fix: add explicit alembic migration step to CI pipeline

Entrypoint migration can fail silently. Now runs explicitly:
  1. Wait for postgres
  2. alembic upgrade head (explicit, fails the deploy if broken)
  3. Wait for backend healthy

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

.gitea/workflows/deploy.yml

@@ -49,6 +49,9 @@ jobs:
               sleep 2
             done
 
+            echo "=== Running database migrations ==="
+            docker compose exec -T backend alembic upgrade head
+
             echo "=== Waiting for backend healthy ==="
             for i in $(seq 1 20); do
               if curl -sf http://localhost:8001/health >/dev/null 2>&1; then