Add background sync worker and complete local-first architecture

Implements automatic background synchronization every 5 minutes:
- Worker pushes pending changes to server (PushPendingChanges)
- Worker pulls new pricelists (SyncPricelistsIfNeeded)
- Graceful shutdown with context cancellation
- Automatic online/offline detection via DB ping

New files:
- internal/services/sync/worker.go - Background sync worker
- internal/services/local_configuration.go - Local-first CRUD
- internal/localdb/converters.go - MariaDB ↔ SQLite converters

Extended sync infrastructure:
- Pending changes queue (pending_changes table)
- Push/pull sync endpoints (/api/sync/push, /pending)
- ConfigurationGetter interface for handler compatibility
- LocalConfigurationService replaces ConfigurationService

All configuration operations now run through SQLite with automatic
background sync to MariaDB when online. Phase 2.5 nearly complete.

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

internal/localdb/converters.go

@@ -0,0 +1,161 @@
+package localdb
+
+import (
+	"time"
+
+	"git.mchus.pro/mchus/quoteforge/internal/models"
+)
+
+// ConfigurationToLocal converts models.Configuration to LocalConfiguration
+func ConfigurationToLocal(cfg *models.Configuration) *LocalConfiguration {
+	items := make(LocalConfigItems, len(cfg.Items))
+	for i, item := range cfg.Items {
+		items[i] = LocalConfigItem{
+			LotName:   item.LotName,
+			Quantity:  item.Quantity,
+			UnitPrice: item.UnitPrice,
+		}
+	}
+
+	local := &LocalConfiguration{
+		UUID:           cfg.UUID,
+		Name:           cfg.Name,
+		Items:          items,
+		TotalPrice:     cfg.TotalPrice,
+		CustomPrice:    cfg.CustomPrice,
+		Notes:          cfg.Notes,
+		IsTemplate:     cfg.IsTemplate,
+		ServerCount:    cfg.ServerCount,
+		CreatedAt:      cfg.CreatedAt,
+		UpdatedAt:      time.Now(),
+		SyncStatus:     "pending",
+		OriginalUserID: cfg.UserID,
+	}
+
+	if cfg.ID > 0 {
+		serverID := cfg.ID
+		local.ServerID = &serverID
+	}
+
+	return local
+}
+
+// LocalToConfiguration converts LocalConfiguration to models.Configuration
+func LocalToConfiguration(local *LocalConfiguration) *models.Configuration {
+	items := make(models.ConfigItems, len(local.Items))
+	for i, item := range local.Items {
+		items[i] = models.ConfigItem{
+			LotName:   item.LotName,
+			Quantity:  item.Quantity,
+			UnitPrice: item.UnitPrice,
+		}
+	}
+
+	cfg := &models.Configuration{
+		UUID:        local.UUID,
+		UserID:      local.OriginalUserID,
+		Name:        local.Name,
+		Items:       items,
+		TotalPrice:  local.TotalPrice,
+		CustomPrice: local.CustomPrice,
+		Notes:       local.Notes,
+		IsTemplate:  local.IsTemplate,
+		ServerCount: local.ServerCount,
+		CreatedAt:   local.CreatedAt,
+	}
+
+	if local.ServerID != nil {
+		cfg.ID = *local.ServerID
+	}
+
+	return cfg
+}
+
+// PricelistToLocal converts models.Pricelist to LocalPricelist
+func PricelistToLocal(pl *models.Pricelist) *LocalPricelist {
+	name := pl.Notification
+	if name == "" {
+		name = pl.Version
+	}
+
+	return &LocalPricelist{
+		ServerID:  pl.ID,
+		Version:   pl.Version,
+		Name:      name,
+		CreatedAt: pl.CreatedAt,
+		SyncedAt:  time.Now(),
+		IsUsed:    false,
+	}
+}
+
+// LocalToPricelist converts LocalPricelist to models.Pricelist
+func LocalToPricelist(local *LocalPricelist) *models.Pricelist {
+	return &models.Pricelist{
+		ID:           local.ServerID,
+		Version:      local.Version,
+		Notification: local.Name,
+		CreatedAt:    local.CreatedAt,
+		IsActive:     true,
+	}
+}
+
+// PricelistItemToLocal converts models.PricelistItem to LocalPricelistItem
+func PricelistItemToLocal(item *models.PricelistItem, localPricelistID uint) *LocalPricelistItem {
+	return &LocalPricelistItem{
+		PricelistID: localPricelistID,
+		LotName:     item.LotName,
+		Price:       item.Price,
+	}
+}
+
+// LocalToPricelistItem converts LocalPricelistItem to models.PricelistItem
+func LocalToPricelistItem(local *LocalPricelistItem, serverPricelistID uint) *models.PricelistItem {
+	return &models.PricelistItem{
+		ID:          local.ID,
+		PricelistID: serverPricelistID,
+		LotName:     local.LotName,
+		Price:       local.Price,
+	}
+}
+
+// ComponentToLocal converts models.LotMetadata to LocalComponent
+func ComponentToLocal(meta *models.LotMetadata) *LocalComponent {
+	var lotDesc string
+	var category string
+
+	if meta.Lot != nil {
+		lotDesc = meta.Lot.LotDescription
+	}
+
+	// Extract category from lot_name (e.g., "CPU_AMD_9654" -> "CPU")
+	if len(meta.LotName) > 0 {
+		for i, ch := range meta.LotName {
+			if ch == '_' {
+				category = meta.LotName[:i]
+				break
+			}
+		}
+	}
+
+	return &LocalComponent{
+		LotName:        meta.LotName,
+		LotDescription: lotDesc,
+		Category:       category,
+		Model:          meta.Model,
+		CurrentPrice:   meta.CurrentPrice,
+		SyncedAt:       time.Now(),
+	}
+}
+
+// LocalToComponent converts LocalComponent to models.LotMetadata
+func LocalToComponent(local *LocalComponent) *models.LotMetadata {
+	return &models.LotMetadata{
+		LotName:      local.LotName,
+		Model:        local.Model,
+		CurrentPrice: local.CurrentPrice,
+		Lot: &models.Lot{
+			LotName:        local.LotName,
+			LotDescription: local.LotDescription,
+		},
+	}
+}