test: raise core-package unit coverage to 90% (#98)

Raises statement coverage of the core packages (all of `internal/` except the interactive `tui/`, plus `pkg/`) from **8.7% to 90.1%**.

## Approach
- **Pure-go unit tests** for all providers, virtual mergers, classifier, config, auth, models, and the API client (httptest).
- **Testcontainers-backed** tests (new `internal/testsupport` helper: Postgres/Redis/MinIO, Ryuk disabled) for database, storage, cache, the proxy engine, the GC, and a full-stack `server` test that drives the whole HTTP API. These `t.Skip` when Docker is absent so `go test` still runs locally without it.

## Measuring
```
go test -coverpkg=./internal/...,./pkg/... -coverprofile=cover.out ./internal/... ./pkg/...
grep -v /internal/tui/ cover.out | go tool cover -func=/dev/stdin | tail -1   # 90.1%
```
Run with `-p 1` (containers are heavy).

## Notes
- The interactive `tui/` package and `cmd/main` are excluded from the target per the agreed scope.
- Some defensive error branches are covered via fault injection (closed DB pool, killing MinIO mid-upload).

Reviewed-on: #98
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/gc/gc_integration_test.go

@@ -0,0 +1,114 @@
+package gc
+
+import (
+	"bytes"
+	"context"
+	"os"
+	"testing"
+	"time"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+	"git.unkin.net/unkin/artifactapi/internal/storage"
+	"git.unkin.net/unkin/artifactapi/internal/testsupport"
+)
+
+var (
+	testDB    *database.DB
+	testStore *storage.S3
+)
+
+func TestMain(m *testing.M) {
+	ctx := context.Background()
+	dsn, termPG, err := testsupport.StartPostgres(ctx)
+	if err != nil {
+		os.Exit(m.Run())
+	}
+	minio, termMinio, err := testsupport.StartMinio(ctx)
+	if err != nil {
+		termPG()
+		os.Exit(m.Run())
+	}
+	db, err := database.New(dsn)
+	if err != nil {
+		panic(err)
+	}
+	var s3 *storage.S3
+	for i := 0; i < 20; i++ {
+		if s3, err = storage.NewS3(minio.Endpoint, minio.AccessKey, minio.SecretKey, "gc-test", false, ""); err == nil {
+			break
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+	if err != nil {
+		panic(err)
+	}
+	testDB = db
+	testStore = s3
+
+	code := m.Run()
+	db.Close()
+	termMinio()
+	termPG()
+	if code != 0 {
+		os.Exit(code)
+	}
+}
+
+func TestSweepDeletesOldOrphan(t *testing.T) {
+	if testDB == nil {
+		t.Skip("Docker unavailable")
+	}
+	ctx := context.Background()
+	hash := "sha256:gcorphan"
+	key := storage.BlobKey("gcorphan")
+
+	if err := testStore.Upload(ctx, key, bytes.NewReader([]byte("orphan")), 6, "application/octet-stream"); err != nil {
+		t.Fatal(err)
+	}
+	if err := testDB.UpsertBlob(ctx, hash, key, 6, "application/octet-stream"); err != nil {
+		t.Fatal(err)
+	}
+	// Age the blob past the grace period.
+	if _, err := testDB.Pool.Exec(ctx, `UPDATE blobs SET created_at = now() - interval '2 hours' WHERE content_hash = $1`, hash); err != nil {
+		t.Fatal(err)
+	}
+
+	c := New(testDB, testStore, time.Hour)
+	c.sweep(ctx)
+
+	if exists, _ := testStore.Exists(ctx, key); exists {
+		t.Error("expected orphan object deleted from store")
+	}
+	orphans, _ := testDB.FindOrphanedBlobs(ctx, 0)
+	for _, b := range orphans {
+		if b.ContentHash == hash {
+			t.Error("expected orphan blob row deleted")
+		}
+	}
+}
+
+func TestSweepNoOrphans(t *testing.T) {
+	if testDB == nil {
+		t.Skip("Docker unavailable")
+	}
+	// A sweep with nothing to collect should be a clean no-op.
+	New(testDB, testStore, time.Hour).sweep(context.Background())
+}
+
+func TestRunStopsOnContextCancel(t *testing.T) {
+	if testDB == nil {
+		t.Skip("Docker unavailable")
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() {
+		New(testDB, testStore, time.Hour).Run(ctx)
+		close(done)
+	}()
+	cancel()
+	select {
+	case <-done:
+	case <-time.After(5 * time.Second):
+		t.Fatal("Run did not return after context cancel")
+	}
+}