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/middleware/offline.go

@@ -4,17 +4,17 @@ import (
 	"log/slog"
 
 	"github.com/gin-gonic/gin"
+	"git.mchus.pro/mchus/quoteforge/internal/db"
 	"git.mchus.pro/mchus/quoteforge/internal/localdb"
-	"gorm.io/gorm"
 )
 
 // OfflineDetector creates middleware that detects offline mode
 // Sets context values:
 //   - "is_offline" (bool) - true if MariaDB is unavailable
 //   - "localdb" (*localdb.LocalDB) - local database instance for fallback
-func OfflineDetector(mariaDB *gorm.DB, local *localdb.LocalDB) gin.HandlerFunc {
+func OfflineDetector(connMgr *db.ConnectionManager, local *localdb.LocalDB) gin.HandlerFunc {
 	return func(c *gin.Context) {
-		isOffline := !checkMariaDBOnline(mariaDB)
+		isOffline := !connMgr.IsOnline()
 
 		// Set context values for handlers
 		c.Set("is_offline", isOffline)
@@ -27,17 +27,3 @@ func OfflineDetector(mariaDB *gorm.DB, local *localdb.LocalDB) gin.HandlerFunc {
 		c.Next()
 	}
 }
-
-// checkMariaDBOnline checks if MariaDB is accessible
-func checkMariaDBOnline(mariaDB *gorm.DB) bool {
-	sqlDB, err := mariaDB.DB()
-	if err != nil {
-		return false
-	}
-
-	if err := sqlDB.Ping(); err != nil {
-		return false
-	}
-
-	return true
-}