fix(ci): serialize integration test packages (-p 1) to stop TRUNCATE deadlocks

First CI integration run proved the act_runner pattern works (service
discovery, migrate, exactly-one guard all functioned) but ~150 tests
failed with `dbtest.ResetDB truncate: deadlock detected (40P01)` plus
cascading FK/dup-key symptoms. Root cause: `go test ./...` runs package
binaries concurrently (default -p = NumCPU); every integration package
TRUNCATEs the single shared minstrel_test DB, so concurrent truncates
deadlock and half-seeded fixtures violate FKs. The documented local
invocation is `go test -p 1 ./...` for exactly this reason. Serialize
package execution in both the CI step and `make test-integration`.

Genuine (non-concurrency) failures will remain after this — never caught
before because integration tests never ran in CI. Triaged next.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Makefile

@@ -17,7 +17,9 @@ test-short:
 test-integration:
 	docker compose up -d postgres
 	-docker compose exec -T postgres createdb -U minstrel minstrel_test
-	MINSTREL_TEST_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel_test?sslmode=disable go test -race ./...
+	# -p 1: integration packages share one test DB and each TRUNCATEs it;
+	# concurrent package binaries deadlock on TRUNCATE. Serialize packages.
+	MINSTREL_TEST_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel_test?sslmode=disable go test -p 1 -race ./...
 
 lint:
 	golangci-lint run ./...