feat: add ConnectionManager for lazy database connections

Introduced ConnectionManager to support offline-first architecture:

- New internal/db/connection.go with thread-safe connection management
- Lazy connection establishment (5s timeout, 10s cooldown)
- Automatic ping caching (30s interval) to avoid excessive checks
- Updated middleware/offline.go to use ConnectionManager.IsOnline()
- Updated sync/worker.go to use ConnectionManager instead of direct DB

This enables the application to start without MariaDB and gracefully
handle offline/online transitions.

Part of Phase 2.5: Full Offline Mode

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

internal/services/sync/worker.go

@@ -5,23 +5,23 @@ import (
 	"log/slog"
 	"time"
 
-	"gorm.io/gorm"
+	"git.mchus.pro/mchus/quoteforge/internal/db"
 )
 
 // Worker performs background synchronization at regular intervals
 type Worker struct {
 	service  *Service
-	db       *gorm.DB
+	connMgr  *db.ConnectionManager
 	interval time.Duration
 	logger   *slog.Logger
 	stopCh   chan struct{}
 }
 
 // NewWorker creates a new background sync worker
-func NewWorker(service *Service, db *gorm.DB, interval time.Duration) *Worker {
+func NewWorker(service *Service, connMgr *db.ConnectionManager, interval time.Duration) *Worker {
 	return &Worker{
 		service:  service,
-		db:       db,
+		connMgr:  connMgr,
 		interval: interval,
 		logger:   slog.Default(),
 		stopCh:   make(chan struct{}),
@@ -30,11 +30,7 @@ func NewWorker(service *Service, db *gorm.DB, interval time.Duration) *Worker {
 
 // isOnline checks if the database connection is available
 func (w *Worker) isOnline() bool {
-	sqlDB, err := w.db.DB()
-	if err != nil {
-		return false
-	}
-	return sqlDB.Ping() == nil
+	return w.connMgr.IsOnline()
 }
 
 // Start begins the background sync loop in a goroutine