Override alembic DB URL from DATABASE_URL env var

Fixes migrations inside Docker container where postgres is at postgres:5432,
not localhost:5453 as hardcoded in alembic.ini.

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

backend/app/db/migrations/env.py

@@ -1,4 +1,5 @@
 import asyncio
+import os
 from logging.config import fileConfig
 
 from alembic import context
@@ -15,6 +16,9 @@ config = context.config
 if config.config_file_name is not None:
     fileConfig(config.config_file_name)
 
+if db_url := os.environ.get("DATABASE_URL"):
+    config.set_main_option("sqlalchemy.url", db_url)
+
 target_metadata = Base.metadata