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/db/postgresql/models.go

@@ -8,6 +8,49 @@ import (
 	"database/sql"
 )
 
+type Collection struct {
+	ID            string `json:"id"`
+	SiteID        string `json:"site_id"`
+	ContainerHtml string `json:"container_html"`
+	CreatedAt     int64  `json:"created_at"`
+	UpdatedAt     int64  `json:"updated_at"`
+	LastEditedBy  string `json:"last_edited_by"`
+}
+
+type CollectionItem struct {
+	ItemID       string `json:"item_id"`
+	CollectionID string `json:"collection_id"`
+	SiteID       string `json:"site_id"`
+	TemplateID   int32  `json:"template_id"`
+	HtmlContent  string `json:"html_content"`
+	Position     int32  `json:"position"`
+	CreatedAt    int64  `json:"created_at"`
+	UpdatedAt    int64  `json:"updated_at"`
+	LastEditedBy string `json:"last_edited_by"`
+}
+
+type CollectionItemVersion struct {
+	VersionID    int32  `json:"version_id"`
+	ItemID       string `json:"item_id"`
+	CollectionID string `json:"collection_id"`
+	SiteID       string `json:"site_id"`
+	HtmlContent  string `json:"html_content"`
+	TemplateID   int32  `json:"template_id"`
+	Position     int32  `json:"position"`
+	CreatedAt    int64  `json:"created_at"`
+	CreatedBy    string `json:"created_by"`
+}
+
+type CollectionTemplate struct {
+	TemplateID   int32  `json:"template_id"`
+	CollectionID string `json:"collection_id"`
+	SiteID       string `json:"site_id"`
+	Name         string `json:"name"`
+	HtmlTemplate string `json:"html_template"`
+	IsDefault    bool   `json:"is_default"`
+	CreatedAt    int64  `json:"created_at"`
+}
+
 type Content struct {
 	ID               string         `json:"id"`
 	SiteID           string         `json:"site_id"`