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_dext_manual_send.py

@@ -13,7 +13,7 @@ async def migrate():
             await conn.execute(text(
                 """
                 ALTER TABLE kitchen_settings
-                ADD COLUMN dext_manual_send_enabled BOOLEAN DEFAULT TRUE
+                ADD COLUMN IF NOT EXISTS dext_manual_send_enabled BOOLEAN DEFAULT TRUE
                 """
             ))
             print("✓ Added dext_manual_send_enabled column")