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_kitchen_details.py

@@ -23,7 +23,7 @@ async def migrate():
         for col_name, col_type in COLUMNS:
             try:
                 await conn.execute(text(
-                    f"ALTER TABLE kitchen_settings ADD COLUMN {col_name} {col_type}"
+                    f"ALTER TABLE kitchen_settings ADD COLUMN IF NOT EXISTS {col_name} {col_type}"
                 ))
                 print(f"+ Added {col_name} column to kitchen_settings")
             except Exception as e: