feat(database): full-page database conversion (Notion-style)

- Migration: add collection_id column to pages table
- POST /api/pages/{id}/convert-to-database: transforms page into database
- GET /api/collections/{id}/table-data: returns props + pages for table view
- POST /api/collections/{id}/pages: creates new row in collection
- New page_editor_collection.html template for database view
- _database_table.html: Notion-style table with columns, rows, New Page, Add Property
- Alpine.js component for cell editing, new page creation, property mgmt
- CSS: complete database table styling (view bar, table, cells, modals, dropdowns)
- Frontend: createDatabase() now converts page to full-page DB (was inline embed)

app/db.py

@@ -487,6 +487,12 @@ def init_db():
             pass
         conn.commit()
 
+        # v4.6.0: Add collection_id to pages (page ↔ collection link for full-page DBs)
+        try:
+            conn.execute("ALTER TABLE pages ADD COLUMN collection_id INTEGER REFERENCES collections(id)")
+        except sqlite3.OperationalError:
+            pass
+
         # v4.2.0: Collection Templates (enhanced) + Dashboards
         # Add description, is_recurring, recurrence_rule to page_templates
         try: