fix(ci): drop postgres host port mapping (multi-runner port collision)

With 3 Gitea Actions runners on the same homelab box, two simultaneous
backend (or backend + e2e) jobs both try to bind 0.0.0.0:5432 for their
postgres service containers. The second fails with:

  failed to set up container networking: ... Bind for 0.0.0.0:5432
  failed: port is already allocated

The host-port mapping isn't actually needed — the workflow uses
\`DATABASE_URL: postgresql+asyncpg://...@postgres:5432/...\` (hostname
\`postgres\` is the service container's docker-network DNS name).
The tests run inside the act container which is on the same docker
network, so they reach postgres without going through the host.

Removing \`ports: 5432:5432\` from both backend and e2e job service
definitions lets multiple postgres services run in parallel on
different docker networks without colliding on the host.

Surfaced when PR #150 ran in parallel with another job after the
multi-runner setup. Backend instant-failed in 2s on the docker run.

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

.gitea/workflows/ci.yml

@@ -17,8 +17,11 @@ jobs:
           POSTGRES_USER: postgres
           POSTGRES_PASSWORD: postgres
           POSTGRES_DB: resolutionflow_test
-        ports:
-          - 5432:5432
+        # No host port mapping. Tests connect to `postgres:5432` (the service
+        # container's docker-network DNS name), not `localhost:5432`. With
+        # multiple Gitea runners on the same homelab box, host-port mapping
+        # would race — two backend/e2e jobs both binding 0.0.0.0:5432 → the
+        # second fails with "port is already allocated".
         options: >-
           --health-cmd pg_isready
           --health-interval 10s
@@ -136,8 +139,11 @@ jobs:
           POSTGRES_USER: postgres
           POSTGRES_PASSWORD: postgres
           POSTGRES_DB: resolutionflow_test
-        ports:
-          - 5432:5432
+        # No host port mapping. Tests connect to `postgres:5432` (the service
+        # container's docker-network DNS name), not `localhost:5432`. With
+        # multiple Gitea runners on the same homelab box, host-port mapping
+        # would race — two backend/e2e jobs both binding 0.0.0.0:5432 → the
+        # second fails with "port is already allocated".
         options: >-
           --health-cmd pg_isready
           --health-interval 10s