Let two backend CI runs share a runner host

The job kept the fixed container names and the fixed 5432 that compose.ci.yml
resets for every other job, so a second run could not start. Layering that
file leaves the db on a port Docker assigns, which the suite is told about,
and the project name is now per run so one teardown cannot take another run's
database with it.

docker/compose.ci.yml

@@ -18,7 +18,11 @@ services:
 
   db:
     container_name: !reset null
-    ports: !reset []
+    # The one service a job may need from the host: test-backend.yml runs pytest
+    # on the runner. Host side left to Docker, so concurrent jobs get a port
+    # each — `docker compose port db 5432` says which.
+    ports: !override
+      - "5432"
 
   redis:
     container_name: !reset null