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/api/v2/local_fault_test.go

@@ -0,0 +1,88 @@
+package v2
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+	"git.unkin.net/unkin/artifactapi/internal/storage"
+	"git.unkin.net/unkin/artifactapi/internal/testsupport"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+// TestLocalUploadStoreFailure covers the upload handlers' store-error branches
+// by killing the object store after a successful upload.
+func TestLocalUploadStoreFailure(t *testing.T) {
+	if testDSN == "" {
+		t.Skip("Docker unavailable")
+	}
+	ctx := context.Background()
+	db, err := database.New(testDSN)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	conn, termMinio, err := testsupport.StartMinio(ctx)
+	if err != nil {
+		t.Skip("minio unavailable")
+	}
+	var store *storage.S3
+	for i := 0; i < 20; i++ {
+		if store, err = storage.NewS3(conn.Endpoint, conn.AccessKey, conn.SecretKey, "fault", false, ""); err == nil {
+			break
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+	if err != nil {
+		termMinio()
+		t.Fatal(err)
+	}
+
+	for _, pt := range []models.PackageType{models.PackageGeneric, models.PackagePyPI} {
+		if err := db.CreateRemote(ctx, &models.Remote{Name: "fault-" + string(pt), PackageType: pt, RepoType: models.RepoTypeLocal}); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	h := NewLocalHandler(db, store)
+	router := chi.NewRouter()
+	router.Route("/remotes/{name}/files", func(r chi.Router) {
+		r.Put("/*", h.Routes().ServeHTTP)
+	})
+	srv := httptest.NewServer(router)
+	defer srv.Close()
+
+	put := func(name, path, body string) int {
+		rq, _ := http.NewRequest("PUT", srv.URL+"/remotes/"+name+"/files/"+path, strings.NewReader(body))
+		resp, err := http.DefaultClient.Do(rq)
+		if err != nil {
+			t.Fatalf("put: %v", err)
+		}
+		resp.Body.Close()
+		return resp.StatusCode
+	}
+
+	// Sanity: uploads succeed while the store is up.
+	if c := put("fault-generic", "ok.bin", "data"); c != 201 {
+		t.Fatalf("generic upload while up = %d", c)
+	}
+	if c := put("fault-pypi", "foo-1.0-py3-none-any.whl", "wheel"); c != 201 {
+		t.Fatalf("pypi upload while up = %d", c)
+	}
+
+	// Kill the store; subsequent CAS.Store calls fail -> 500.
+	termMinio()
+	if c := put("fault-generic", "after.bin", "data"); c != 500 {
+		t.Errorf("generic upload after store down = %d, want 500", c)
+	}
+	if c := put("fault-pypi", "bar-1.0-py3-none-any.whl", "wheel"); c != 500 {
+		t.Errorf("pypi upload after store down = %d, want 500", c)
+	}
+}