fix: run alembic migration in one-shot container before backend start

- Use 'docker compose run --rm backend alembic upgrade head' to run
  migration in a throwaway container (doesn't need backend running)
- Restore entrypoint migration fallback (don't block startup)
- Restart backend after migration to pick up schema changes
- Remove --no-cache (too slow, was causing timeouts)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -29,8 +29,8 @@ jobs:
             git fetch origin production-deployment
             git reset --hard origin/production-deployment
 
-            echo "=== Building images (no cache) ==="
-            docker compose build --no-cache backend frontend
+            echo "=== Building images ==="
+            docker compose build backend frontend
 
             echo "=== Starting services ==="
             docker compose up -d --remove-orphans
@@ -49,8 +49,11 @@ jobs:
               sleep 2
             done
 
-            echo "=== Running database migrations ==="
-            docker compose exec -T backend alembic upgrade head
+            echo "=== Running database migrations (one-shot container) ==="
+            docker compose run --rm backend alembic upgrade head
+
+            echo "=== Restarting backend to pick up migration ==="
+            docker compose restart backend
 
             echo "=== Waiting for backend healthy ==="
             for i in $(seq 1 20); do