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_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
 	}