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/proxy/classifier_extra_test.go

@@ -0,0 +1,52 @@
+package proxy
+
+import (
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+	_ "git.unkin.net/unkin/artifactapi/internal/provider/generic"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestClassifierBranches(t *testing.T) {
+	gp, err := provider.Get(models.PackageGeneric)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c := NewClassifier(gp)
+
+	if c.Classify(models.Remote{Blocklist: []string{`\.exe$`}}, "x.exe") != ClassDenied {
+		t.Error("blocklist match should be denied")
+	}
+	// Allowlist present but path doesn't match -> denied.
+	allow := models.Remote{Patterns: []string{`^allowed/`}}
+	if c.Classify(allow, "other/x") != ClassDenied {
+		t.Error("non-allowlisted path should be denied")
+	}
+	if c.Classify(allow, "allowed/x") != ClassImmutable {
+		t.Error("allowlisted generic path should be immutable")
+	}
+	if c.Classify(models.Remote{MutablePatterns: []string{`index$`}}, "a/index") != ClassMutable {
+		t.Error("mutable pattern override failed")
+	}
+	if c.Classify(models.Remote{ImmutablePatterns: []string{`\.bin$`}}, "a.bin") != ClassImmutable {
+		t.Error("immutable pattern failed")
+	}
+	// An invalid regex is skipped (not treated as a match) rather than denying.
+	if c.Classify(models.Remote{Blocklist: []string{`[invalid`}}, "anything") == ClassDenied {
+		t.Error("invalid blocklist regex should be skipped, not deny everything")
+	}
+}
+
+func TestClassificationString(t *testing.T) {
+	for c, want := range map[Classification]string{
+		ClassImmutable:     "immutable",
+		ClassMutable:       "mutable",
+		ClassDenied:        "denied",
+		Classification(99): "unknown",
+	} {
+		if c.String() != want {
+			t.Errorf("Classification(%d).String() = %q, want %q", c, c.String(), want)
+		}
+	}
+}