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_portion_name.py

@@ -1,22 +1,22 @@
-"""Add sambapos_portion_name column to recipes table."""
-import logging
-from sqlalchemy import text
-from database import engine
-
-logger = logging.getLogger(__name__)
-
-
-async def migrate():
-    async with engine.begin() as conn:
-        result = await conn.execute(text(
-            "SELECT column_name FROM information_schema.columns "
-            "WHERE table_name = 'recipes' AND column_name = 'sambapos_portion_name'"
-        ))
-        if result.scalar_one_or_none():
-            logger.info("recipes.sambapos_portion_name already exists, skipping")
-            return
-
-        await conn.execute(text(
-            "ALTER TABLE recipes ADD COLUMN sambapos_portion_name VARCHAR(255) DEFAULT NULL"
-        ))
-        logger.info("Added recipes.sambapos_portion_name column")
+"""Add sambapos_portion_name column to recipes table."""
+import logging
+from sqlalchemy import text
+from database import engine
+
+logger = logging.getLogger(__name__)
+
+
+async def migrate():
+    async with engine.begin() as conn:
+        result = await conn.execute(text(
+            "SELECT column_name FROM information_schema.columns "
+            "WHERE table_name = 'recipes' AND column_name = 'sambapos_portion_name'"
+        ))
+        if result.scalar_one_or_none():
+            logger.info("recipes.sambapos_portion_name already exists, skipping")
+            return
+
+        await conn.execute(text(
+            "ALTER TABLE recipes ADD COLUMN IF NOT EXISTS sambapos_portion_name VARCHAR(255) DEFAULT NULL"
+        ))
+        logger.info("Added recipes.sambapos_portion_name column")