Give the staging database its own name

The staging database now runs as qrmaster_test instead of reusing the production name.
Container and volume already kept the two apart, but a hand-typed psql session against
two databases both called `qrmaster` looks identical on either side - the distinct name is
what makes the wrong window obvious before a DELETE lands in it.

The base compose file hardcodes `pg_isready -d qrmaster` in the db healthcheck, so the
overlay has to override the probe as well. Without it the container stays unhealthy and web
never starts, because it waits on service_healthy.

Verified against `docker compose config`: staging resolves to qrmaster_test in POSTGRES_DB,
DATABASE_URL and the healthcheck, while production still resolves to qrmaster.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker-compose.test.yml

@@ -12,6 +12,12 @@ services:
     container_name: qrmaster-test-db
     # Production already publishes 5435 on the host.
     ports: !reset []
+    # The base file hardcodes the database name in the probe. Staging uses its own name so a
+    # hand-typed psql session makes it obvious which instance you are on - without this
+    # override the probe would check a database that does not exist, the container would stay
+    # unhealthy and web (depends_on: service_healthy) would never start.
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres -d qrmaster_test"]
     # Only on the internal network. `db` and `redis` are network aliases assigned per
     # compose project, so leaving them on the shared external network would make `db`
     # resolve to two containers and this stack could reach the production database.