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/provider/goproxy/goproxy_extra_test.go

@@ -0,0 +1,27 @@
+package goproxy
+
+import (
+	"context"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestGoProxyURLAuthRewrite(t *testing.T) {
+	p := &Provider{}
+	if got := p.UpstreamURL(models.Remote{BaseURL: "https://proxy.golang.org/"}, "/mod/@v/list"); got != "https://proxy.golang.org/mod/@v/list" {
+		t.Errorf("upstream url %q", got)
+	}
+	if out, err := p.RewriteResponse([]byte("x"), models.Remote{}, "http://p"); out != nil || err != nil {
+		t.Error("goproxy never rewrites")
+	}
+	if h, _ := p.AuthHeaders(context.Background(), models.Remote{Username: "u", Password: "p"}); h.Get("Authorization") == "" {
+		t.Error("expected basic auth header")
+	}
+	if got := p.ContentType("mod/@v/v1.0.0.info"); got != "application/json" {
+		t.Errorf("info content type %q", got)
+	}
+	if got := p.ContentType("mod/@v/v1.0.0.mod"); got != "text/plain" {
+		t.Errorf("mod content type %q", got)
+	}
+}