fix: address review findings for backup management

- HIGH: Add sync.Mutex to backup Engine to prevent concurrent
  backup/restore operations
- HIGH: Restore uses io.Copy instead of ReadFile to avoid OOM on
  large databases
- HIGH: Send HTTP response before closing DB during restore, then
  perform destructive operations in a goroutine
- HIGH: Create pre-restore safety backup before overwriting database
- HIGH: Autobackup cron reschedules dynamically when settings change
  via callback pattern (same as DNS provider changes)

internal/api/backups.go

@@ -1,9 +1,12 @@
 package api
 
 import (
+	"io"
+	"log/slog"
 	"net/http"
 	"os"
 	"path/filepath"
+	"time"
 
 	"github.com/alexei/docker-watcher/internal/store"
 	"github.com/go-chi/chi/v5"
@@ -93,7 +96,7 @@ func (s *Server) deleteBackup(w http.ResponseWriter, r *http.Request) {
 }
 
 // restoreBackup handles POST /api/backups/{id}/restore.
-// This replaces the current database with the backup. The server should be restarted after.
+// This replaces the current database with the backup and triggers a graceful shutdown.
 func (s *Server) restoreBackup(w http.ResponseWriter, r *http.Request) {
 	if s.backupEngine == nil {
 		respondError(w, http.StatusServiceUnavailable, "backup engine not initialized")
@@ -107,36 +110,62 @@ func (s *Server) restoreBackup(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Read the backup file.
-	backupData, err := os.ReadFile(restorePath)
-	if err != nil {
-		respondError(w, http.StatusInternalServerError, "failed to read backup file: "+err.Error())
-		return
+	// Create a safety backup before restore so the user can undo if needed.
+	if _, err := s.backupEngine.CreateBackup("pre-restore"); err != nil {
+		slog.Warn("failed to create pre-restore backup", "error", err)
 	}
 
-	// Close the current database to release locks.
-	if err := s.store.Close(); err != nil {
-		respondError(w, http.StatusInternalServerError, "failed to close database: "+err.Error())
-		return
-	}
-
-	// Write backup over the main database file.
-	if err := os.WriteFile(s.dbPath, backupData, 0o644); err != nil {
-		respondError(w, http.StatusInternalServerError, "failed to write database: "+err.Error())
-		return
-	}
-
-	// Remove WAL and SHM files to ensure clean state.
-	os.Remove(s.dbPath + "-wal")
-	os.Remove(s.dbPath + "-shm")
-
+	// Send the response BEFORE closing the DB so the client gets confirmation.
 	respondJSON(w, http.StatusOK, map[string]any{
-		"status":  "restored",
-		"message": "Database restored. The server needs to be restarted to apply changes.",
+		"status":  "restoring",
+		"message": "Database restore initiated. The server will restart shortly.",
 	})
 
-	// Signal the server to shut down gracefully so it can be restarted.
-	if s.shutdownFunc != nil {
-		go s.shutdownFunc()
+	// Flush the response.
+	if f, ok := w.(http.Flusher); ok {
+		f.Flush()
 	}
+
+	// Perform the destructive restore in a goroutine with a brief delay
+	// to allow the HTTP response to be fully sent.
+	go func() {
+		time.Sleep(500 * time.Millisecond)
+
+		// Close the current database to release locks.
+		if err := s.store.Close(); err != nil {
+			slog.Error("restore: failed to close database", "error", err)
+			return
+		}
+
+		// Copy the backup file over the main database using streaming (no full read into memory).
+		src, err := os.Open(restorePath)
+		if err != nil {
+			slog.Error("restore: failed to open backup file", "error", err)
+			return
+		}
+		defer src.Close()
+
+		dst, err := os.Create(s.dbPath)
+		if err != nil {
+			slog.Error("restore: failed to create database file", "error", err)
+			return
+		}
+		defer dst.Close()
+
+		if _, err := io.Copy(dst, src); err != nil {
+			slog.Error("restore: failed to copy backup to database", "error", err)
+			return
+		}
+
+		// Remove WAL and SHM files to ensure clean state.
+		os.Remove(s.dbPath + "-wal")
+		os.Remove(s.dbPath + "-shm")
+
+		slog.Info("restore: database replaced, triggering shutdown")
+
+		// Signal the server to shut down gracefully so it can be restarted.
+		if s.shutdownFunc != nil {
+			s.shutdownFunc()
+		}
+	}()
 }