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

internal/content/client.go

@@ -171,3 +171,20 @@ func (c *HTTPClient) CreateContent(siteID, contentID, htmlContent, originalTempl
 	// This would typically be used in API-driven enhancement scenarios
 	return nil, fmt.Errorf("CreateContent not implemented for HTTPClient - use DatabaseClient for enhancement")
 }
+
+// Collection method stubs - TODO: Implement these for HTTP API
+func (c *HTTPClient) GetCollection(siteID, collectionID string) (*engine.CollectionItem, error) {
+	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
+}
+
+func (c *HTTPClient) CreateCollection(siteID, collectionID, containerHTML, lastEditedBy string) (*engine.CollectionItem, error) {
+	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
+}
+
+func (c *HTTPClient) GetCollectionItems(siteID, collectionID string) ([]engine.CollectionItemWithTemplate, error) {
+	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
+}
+
+func (c *HTTPClient) CreateCollectionTemplate(siteID, collectionID, name, htmlTemplate string, isDefault bool) (*engine.CollectionTemplateItem, error) {
+	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
+}