CI: quote the db-port step so the workflow parses

`cut -d: -f2` inside a plain YAML scalar reads as a mapping key, so
test-backend.yml was not valid YAML and would have been rejected the first time
a runner picked it up. A block scalar takes the whole command as text.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/test-backend.yml

@@ -52,7 +52,8 @@ jobs:
       - name: Resolve the db port
         # Docker assigns it (docker/compose.ci.yml), so concurrent runs get one
         # each. An env var outranks the .env file in the backend settings.
-        run: echo "POSTGRES_PORT=$(docker compose port db 5432 | tail -n1 | cut -d: -f2)" >> "$GITHUB_ENV"
+        run: |
+          echo "POSTGRES_PORT=$(docker compose port db 5432 | tail -n1 | cut -d: -f2)" >> "$GITHUB_ENV"
       - run: uv sync
       - name: Migrate DB and seed the superuser
         run: uv run bash scripts/prestart.sh