fix: SQLite WAL mode + log all swallowed 500 errors

- internal/db/db.go: Use _pragma syntax so modernc.org/sqlite actually
  applies busy_timeout(5000) and journal_mode(WAL). Eliminates SQLITE_BUSY
  500s when concurrent reads hit a writer holding the DELETE-mode lock.
- internal/api/handlers_*.go: Add slog.Error before every writeError with
  StatusInternalServerError so real errors appear in logs (30+ sites across
  handlers_jobs, handlers_machines, handlers_syncpairs, handlers_sshkeys).

internal/api/handlers_jobs.go

@@ -57,6 +57,7 @@ func (h *JobHandler) List(w http.ResponseWriter, r *http.Request) {
 	repo := models.NewJobLogRepository(h.db)
 	jobs, total, err := repo.GetAllFiltered(limit, offset, syncPairID, status, triggerType, from, to)
 	if err != nil {
+		slog.Error("failed to fetch jobs", "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch jobs")
 		return
 	}
@@ -83,6 +84,7 @@ func (h *JobHandler) Get(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch job", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch job")
 		return
 	}
@@ -103,6 +105,7 @@ func (h *JobHandler) Cancel(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch job", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch job")
 		return
 	}
@@ -146,6 +149,7 @@ func (h *JobHandler) TriggerRun(w http.ResponseWriter, r *http.Request) {
 
 	jobID, err := h.engine.CreateJob(pairID, "manual")
 	if err != nil {
+		slog.Error("failed to create job", "pair_id", pairID, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to create job")
 		return
 	}
@@ -162,6 +166,7 @@ func (h *JobHandler) TriggerRun(w http.ResponseWriter, r *http.Request) {
 	jobRepo := models.NewJobRepository(h.db)
 	j, err := jobRepo.GetByID(jobID)
 	if err != nil {
+		slog.Error("failed to fetch created job", "job_id", jobID, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch created job")
 		return
 	}
@@ -184,6 +189,7 @@ func (h *JobHandler) GetLog(w http.ResponseWriter, r *http.Request) {
 	logRepo := models.NewJobLogRepository(h.db)
 	logs, err := logRepo.GetByJobID(id, limit, offset)
 	if err != nil {
+		slog.Error("failed to fetch logs", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch logs")
 		return
 	}
@@ -207,6 +213,7 @@ func (h *JobHandler) DownloadLog(w http.ResponseWriter, r *http.Request) {
 	jobRepo := models.NewJobRepository(h.db)
 	j, err := jobRepo.GetByID(id)
 	if err != nil {
+		slog.Error("failed to fetch job", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch job")
 		return
 	}

internal/api/handlers_machines.go

@@ -30,6 +30,7 @@ func (h *MachineHandler) List(w http.ResponseWriter, r *http.Request) {
 	repo := models.NewMachineRepository(h.db)
 	ms, err := repo.GetAll()
 	if err != nil {
+		slog.Error("failed to fetch machines", "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch machines")
 		return
 	}
@@ -53,6 +54,7 @@ func (h *MachineHandler) Get(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch machine", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch machine")
 		return
 	}
@@ -106,6 +108,7 @@ func (h *MachineHandler) Create(w http.ResponseWriter, r *http.Request) {
 	repo := models.NewMachineRepository(h.db)
 	id, err := repo.Create(m)
 	if err != nil {
+		slog.Error("failed to create machine", "name", req.Name, "host", req.Host, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to create machine")
 		return
 	}
@@ -150,6 +153,7 @@ func (h *MachineHandler) Update(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch machine", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch machine")
 		return
 	}
@@ -170,6 +174,7 @@ func (h *MachineHandler) Update(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if err := repo.Update(existing); err != nil {
+		slog.Error("failed to update machine", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to update machine")
 		return
 	}
@@ -189,6 +194,7 @@ func (h *MachineHandler) TestWoL(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch machine", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch machine")
 		return
 	}
@@ -238,6 +244,7 @@ func (h *MachineHandler) Delete(w http.ResponseWriter, r *http.Request) {
 
 	repo := models.NewMachineRepository(h.db)
 	if err := repo.Delete(id); err != nil {
+		slog.Error("failed to delete machine", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to delete machine")
 		return
 	}

internal/api/handlers_sshkeys.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -32,6 +33,7 @@ func (h *SSHKeyHandler) List(w http.ResponseWriter, r *http.Request) {
 	machineRepo := models.NewMachineRepository(h.db)
 	keys, err := repo.GetAll()
 	if err != nil {
+		slog.Error("failed to fetch ssh keys", "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch ssh keys")
 		return
 	}
@@ -87,6 +89,7 @@ func (h *SSHKeyHandler) Create(w http.ResponseWriter, r *http.Request) {
 
 	keysDir := filepath.Join(h.cfg.SSHDir(), "keys")
 	if err := os.MkdirAll(keysDir, 0700); err != nil {
+		slog.Error("failed to create keys directory", "path", keysDir, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to create keys directory")
 		return
 	}
@@ -97,6 +100,7 @@ func (h *SSHKeyHandler) Create(w http.ResponseWriter, r *http.Request) {
 	if req.Generate {
 		privPath, _, pubKey, fp, err = sshmanager.GenerateKeyPair(req.Label, keysDir)
 		if err != nil {
+			slog.Error("generating key", "label", req.Label, "error", err)
 			writeError(w, http.StatusInternalServerError, fmt.Sprintf("generating key: %v", err))
 			return
 		}
@@ -113,6 +117,7 @@ func (h *SSHKeyHandler) Create(w http.ResponseWriter, r *http.Request) {
 	repo := models.NewSSHKeyRepository(h.db)
 	id, err := repo.Create(req.Label, privPath, pubKey)
 	if err != nil {
+		slog.Error("failed to store ssh key", "label", req.Label, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to store ssh key")
 		return
 	}
@@ -140,6 +145,7 @@ func (h *SSHKeyHandler) Get(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch ssh key", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch ssh key")
 		return
 	}
@@ -195,6 +201,7 @@ func (h *SSHKeyHandler) Delete(w http.ResponseWriter, r *http.Request) {
 		os.Remove(k.PrivateKeyPath + ".pub")
 	}
 	if err := repo.Delete(id); err != nil {
+		slog.Error("failed to delete ssh key", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to delete ssh key")
 		return
 	}
@@ -214,6 +221,7 @@ func (h *SSHKeyHandler) DownloadPrivate(w http.ResponseWriter, r *http.Request)
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch ssh key", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch ssh key")
 		return
 	}
@@ -224,6 +232,7 @@ func (h *SSHKeyHandler) DownloadPrivate(w http.ResponseWriter, r *http.Request)
 
 	data, err := os.ReadFile(k.PrivateKeyPath)
 	if err != nil {
+		slog.Error("failed to read private key", "path", k.PrivateKeyPath, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to read private key")
 		return
 	}

internal/api/handlers_syncpairs.go

@@ -3,6 +3,7 @@ package api
 import (
 	"database/sql"
 	"encoding/json"
+	"log/slog"
 	"net/http"
 	"regexp"
 	"strconv"
@@ -25,6 +26,7 @@ func (h *SyncPairHandler) List(w http.ResponseWriter, r *http.Request) {
 	repo := models.NewSyncPairRepository(h.db)
 	pairs, err := repo.GetAll()
 	if err != nil {
+		slog.Error("failed to fetch sync pairs", "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch sync pairs")
 		return
 	}
@@ -48,6 +50,7 @@ func (h *SyncPairHandler) Get(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch sync pair", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch sync pair")
 		return
 	}
@@ -94,6 +97,7 @@ func (h *SyncPairHandler) Create(w http.ResponseWriter, r *http.Request) {
 	repo := models.NewSyncPairRepository(h.db)
 	id, err := repo.Create(sp)
 	if err != nil {
+		slog.Error("failed to create sync pair", "name", req.Name, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to create sync pair")
 		return
 	}
@@ -131,6 +135,7 @@ func (h *SyncPairHandler) Update(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err != nil {
+		slog.Error("failed to fetch sync pair", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to fetch sync pair")
 		return
 	}
@@ -146,6 +151,7 @@ func (h *SyncPairHandler) Update(w http.ResponseWriter, r *http.Request) {
 	existing.Enabled = req.Enabled
 
 	if err := repo.Update(existing); err != nil {
+		slog.Error("failed to update sync pair", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to update sync pair")
 		return
 	}
@@ -161,6 +167,7 @@ func (h *SyncPairHandler) Delete(w http.ResponseWriter, r *http.Request) {
 
 	repo := models.NewSyncPairRepository(h.db)
 	if err := repo.Delete(id); err != nil {
+		slog.Error("failed to delete sync pair", "id", id, "error", err)
 		writeError(w, http.StatusInternalServerError, "failed to delete sync pair")
 		return
 	}

internal/db/db.go

@@ -24,7 +24,7 @@ func Open(dbPath string) (*DB, error) {
 		return nil, err
 	}
 
-	db, err := sql.Open("sqlite", dbPath+"?_journal_mode=WAL&_foreign_keys=ON&_busy_timeout=5000")
+	db, err := sql.Open("sqlite", dbPath+"?_pragma=busy_timeout(5000)&_pragma=journal_mode(WAL)")
 	if err != nil {
 		return nil, fmt.Errorf("opening db: %w", err)
 	}