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/collections.sql

@@ -0,0 +1,39 @@
+-- Collections table queries
+
+-- name: GetCollection :one
+SELECT id, site_id, container_html, created_at, updated_at, last_edited_by
+FROM collections 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);
+
+-- name: GetAllCollections :many
+SELECT id, site_id, container_html, created_at, updated_at, last_edited_by
+FROM collections 
+WHERE site_id = sqlc.arg(site_id)
+ORDER BY updated_at DESC;
+
+-- name: CreateCollection :one
+INSERT INTO collections (id, site_id, container_html, last_edited_by)
+VALUES (sqlc.arg(id), sqlc.arg(site_id), sqlc.arg(container_html), sqlc.arg(last_edited_by))
+RETURNING id, site_id, container_html, created_at, updated_at, last_edited_by;
+
+-- name: UpdateCollection :one
+UPDATE collections 
+SET container_html = sqlc.arg(container_html), last_edited_by = sqlc.arg(last_edited_by)
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id)
+RETURNING id, site_id, container_html, created_at, updated_at, last_edited_by;
+
+-- name: UpsertCollection :one
+INSERT INTO collections (id, site_id, container_html, last_edited_by)
+VALUES (sqlc.arg(id), sqlc.arg(site_id), sqlc.arg(container_html), sqlc.arg(last_edited_by))
+ON CONFLICT(id, site_id) DO UPDATE SET
+    container_html = EXCLUDED.container_html,
+    last_edited_by = EXCLUDED.last_edited_by
+RETURNING id, site_id, container_html, created_at, updated_at, last_edited_by;
+
+-- name: DeleteCollection :exec
+DELETE FROM collections 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);
+
+-- name: DeleteAllSiteCollections :exec
+DELETE FROM collections 
+WHERE site_id = sqlc.arg(site_id);