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/queries/collection_item_versions.sql

@@ -0,0 +1,31 @@
+-- Collection item versions table queries
+
+-- name: CreateCollectionItemVersion :exec
+INSERT INTO collection_item_versions (item_id, collection_id, site_id, html_content, template_id, position, created_by)
+VALUES (sqlc.arg(item_id), sqlc.arg(collection_id), sqlc.arg(site_id), sqlc.arg(html_content), sqlc.arg(template_id), sqlc.arg(position), sqlc.arg(created_by));
+
+-- name: GetCollectionItemVersionHistory :many
+SELECT version_id, item_id, collection_id, site_id, html_content, template_id, position, created_at, created_by
+FROM collection_item_versions
+WHERE item_id = sqlc.arg(item_id) AND collection_id = sqlc.arg(collection_id) AND site_id = sqlc.arg(site_id)
+ORDER BY created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: GetCollectionItemVersion :one
+SELECT version_id, item_id, collection_id, site_id, html_content, template_id, position, created_at, created_by
+FROM collection_item_versions
+WHERE version_id = sqlc.arg(version_id);
+
+-- name: GetAllCollectionItemVersionsForSite :many
+SELECT 
+    civ.version_id, civ.item_id, civ.collection_id, civ.site_id, civ.html_content, civ.template_id, civ.position, civ.created_at, civ.created_by,
+    ci.html_content as current_html_content, ci.position as current_position
+FROM collection_item_versions civ
+LEFT JOIN collection_items ci ON civ.item_id = ci.item_id AND civ.collection_id = ci.collection_id AND civ.site_id = ci.site_id
+WHERE civ.site_id = sqlc.arg(site_id)
+ORDER BY civ.created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: DeleteOldCollectionItemVersions :exec
+DELETE FROM collection_item_versions 
+WHERE created_at < sqlc.arg(created_before) AND site_id = sqlc.arg(site_id);