Add Phase 2: Local SQLite database with sync functionality

Implements complete offline-first architecture with SQLite caching and MariaDB synchronization.

Key features:
- Local SQLite database for offline operation (data/quoteforge.db)
- Connection settings with encrypted credentials
- Component and pricelist caching with auto-sync
- Sync API endpoints (/api/sync/status, /components, /pricelists, /all)
- Real-time sync status indicator in UI with auto-refresh
- Offline mode detection middleware
- Migration tool for database initialization
- Setup wizard for initial configuration

New components:
- internal/localdb: SQLite repository layer (components, pricelists, sync)
- internal/services/sync: Synchronization service
- internal/handlers/sync: Sync API handlers
- internal/handlers/setup: Setup wizard handlers
- internal/middleware/offline: Offline detection
- cmd/migrate: Database migration tool

UI improvements:
- Setup page for database configuration
- Sync status indicator with online/offline detection
- Warning icons for pending synchronization
- Auto-refresh every 30 seconds

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/models/models.go

@@ -1,6 +1,11 @@
 package models
 
-import "gorm.io/gorm"
+import (
+	"log/slog"
+	"strings"
+
+	"gorm.io/gorm"
+)
 
 // AllModels returns all models for auto-migration
 func AllModels() []interface{} {
@@ -12,12 +17,28 @@ func AllModels() []interface{} {
 		&PriceOverride{},
 		&PricingAlert{},
 		&ComponentUsageStats{},
+		&Pricelist{},
+		&PricelistItem{},
 	}
 }
 
 // Migrate runs auto-migration for all QuoteForge tables
+// Handles MySQL constraint errors gracefully for existing tables
 func Migrate(db *gorm.DB) error {
-	return db.AutoMigrate(AllModels()...)
+	for _, model := range AllModels() {
+		if err := db.AutoMigrate(model); err != nil {
+			// Skip known MySQL constraint errors for existing tables
+			errStr := err.Error()
+			if strings.Contains(errStr, "Can't DROP") ||
+				strings.Contains(errStr, "Duplicate key name") ||
+				strings.Contains(errStr, "check that it exists") {
+				slog.Warn("migration warning (skipped)", "model", model, "error", errStr)
+				continue
+			}
+			return err
+		}
+	}
+	return nil
 }
 
 // SeedCategories inserts default categories if not exist
@@ -49,3 +70,35 @@ func SeedAdminUser(db *gorm.DB, passwordHash string) error {
 	}
 	return db.Create(admin).Error
 }
+
+// EnsureDBUser creates or returns the user corresponding to the database connection username.
+// This is used when RBAC is disabled - configurations are owned by the DB user.
+// Returns the user ID that should be used for all operations.
+func EnsureDBUser(db *gorm.DB, dbUsername string) (uint, error) {
+	if dbUsername == "" {
+		return 0, nil
+	}
+
+	var user User
+	err := db.Where("username = ?", dbUsername).First(&user).Error
+	if err == nil {
+		return user.ID, nil
+	}
+
+	// User doesn't exist, create it
+	user = User{
+		Username:     dbUsername,
+		Email:        dbUsername + "@db.local",
+		PasswordHash: "-", // No password - this is a DB user, not an app user
+		Role:         RoleAdmin,
+		IsActive:     true,
+	}
+
+	if err := db.Create(&user).Error; err != nil {
+		slog.Error("failed to create DB user", "username", dbUsername, "error", err)
+		return 0, err
+	}
+
+	slog.Info("created DB user for configurations", "username", dbUsername, "user_id", user.ID)
+	return user.ID, nil
+}