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/mock.go

@@ -1,6 +1,7 @@
 package content
 
 import (
+	"fmt"
 	"time"
 
 	"github.com/insertr/insertr/internal/engine"
@@ -156,3 +157,20 @@ func (m *MockClient) CreateContent(siteID, contentID, htmlContent, originalTempl
 
 	return &item, nil
 }
+
+// Collection method stubs - TODO: Implement these for mock testing
+func (m *MockClient) GetCollection(siteID, collectionID string) (*engine.CollectionItem, error) {
+	return nil, fmt.Errorf("collection operations not implemented in MockClient")
+}
+
+func (m *MockClient) CreateCollection(siteID, collectionID, containerHTML, lastEditedBy string) (*engine.CollectionItem, error) {
+	return nil, fmt.Errorf("collection operations not implemented in MockClient")
+}
+
+func (m *MockClient) GetCollectionItems(siteID, collectionID string) ([]engine.CollectionItemWithTemplate, error) {
+	return nil, fmt.Errorf("collection operations not implemented in MockClient")
+}
+
+func (m *MockClient) CreateCollectionTemplate(siteID, collectionID, name, htmlTemplate string, isDefault bool) (*engine.CollectionTemplateItem, error) {
+	return nil, fmt.Errorf("collection operations not implemented in MockClient")
+}