ci(tenant-registry): use -coverpkg so server tests count store coverage

The store package has no test files of its own (its API is exercised
end-to-end through the server's eachStore harness against both Memory
and Postgres). Without -coverpkg, store/* shows 0% and drags the
total below the 70% gate even though every store method is run.

-coverpkg=./internal/... routes the instrumentation from server tests
into store + config + server alike.

Refs: M4.2

.gitea/workflows/ci.yaml

@@ -78,9 +78,11 @@ jobs:
 
       - name: test
         # Coverage scoped to ./internal/... — cmd/server is the entrypoint
-        # with signal-handling + bind that isn't worth unit-testing. When
+        # with signal-handling + bind that isn't worth unit-testing.
-        # real integration tests land in M4.1, widen this back to ./...
+        # -coverpkg lets the server tests count their exercise of store/* —
-        run: go test -race -coverprofile=cover.out ./internal/...
+        # store has no test files because every method is exercised end-to-end
+        # via the server's eachStore harness against both Memory and Postgres.
+        run: go test -race -coverpkg=./internal/... -coverprofile=cover.out ./internal/...
 
       - name: coverage gate
         run: |