Implement complete collection persistence with database-backed survival across server restarts

• Add full multi-table schema for collections with normalized design (collections, collection_templates, collection_items, collection_item_versions)
• Implement collection detection and processing in enhancement pipeline for .insertr-add elements
• Add template extraction and storage from existing HTML children with multi-variant support
• Enable collection reconstruction from database on server restart with proper DOM rebuilding
• Extend ContentClient interface with collection operations and full database integration
• Update enhance command to use engine.DatabaseClient for collection persistence support

cmd/enhance.go

@@ -102,7 +102,7 @@ func runEnhance(cmd *cobra.Command, args []string) {
 			log.Fatalf("Failed to initialize database: %v", err)
 		}
 		defer database.Close()
-		client = content.NewDatabaseClient(database)
+		client = engine.NewDatabaseClient(database)
 	} else {
 		fmt.Printf("🧪 No database or API configured, using mock content\n")
 		client = content.NewMockClient()