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_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
 	}