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_sambapos_excluded_items.py

@@ -17,7 +17,7 @@ async def run_migration():
     logger.info("Running SambaPOS excluded items migration...")
 
     # Add column for excluded items (TEXT to allow for many items with long names)
-    sql = "ALTER TABLE kitchen_settings ADD COLUMN sambapos_excluded_items TEXT"
+    sql = "ALTER TABLE kitchen_settings ADD COLUMN IF NOT EXISTS sambapos_excluded_items TEXT"
 
     try:
         async with engine.begin() as conn: