Fix migration transaction poisoning: ADD COLUMN IF NOT EXISTS

Migrations used try/except around ADD COLUMN inside a single engine.begin()
block. When a 'column already exists' error was caught, asyncpg left the
transaction in aborted state, causing all subsequent DDL in the block to fail
with InFailedSQLTransactionError. Replace with IF NOT EXISTS to prevent the
error entirely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/add_admin_restricted_pages.py

@@ -16,7 +16,7 @@ async def run_migration():
     """Add admin_restricted_pages column to kitchen_settings table."""
     logger.info("Running admin restricted pages migration...")
 
-    sql = "ALTER TABLE kitchen_settings ADD COLUMN admin_restricted_pages TEXT"
+    sql = "ALTER TABLE kitchen_settings ADD COLUMN IF NOT EXISTS admin_restricted_pages TEXT"
 
     try:
         async with engine.begin() as conn: