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/docker/docker_extra_test.go

@@ -0,0 +1,53 @@
+package docker
+
+import (
+	"context"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestDockerClassifyBranches(t *testing.T) {
+	p := &Provider{}
+	if p.Classify("library/nginx/tags/list") != provider.Mutable {
+		t.Error("tags/list should be mutable")
+	}
+	if p.Classify("library/nginx/manifests/latest") != provider.Mutable {
+		t.Error("tag manifest should be mutable")
+	}
+	if p.Classify("library/nginx/manifests/sha256:abcdef") != provider.Immutable {
+		t.Error("digest manifest should be immutable")
+	}
+	if p.Classify("library/nginx/blobs/sha256:abc") != provider.Immutable {
+		t.Error("blob should be immutable")
+	}
+}
+
+func TestDockerContentType(t *testing.T) {
+	p := &Provider{}
+	if p.ContentType("x/blobs/sha256:abc") != "application/octet-stream" {
+		t.Error("blob content type")
+	}
+	if p.ContentType("x/manifests/latest") != "application/vnd.docker.distribution.manifest.v2+json" {
+		t.Error("manifest content type")
+	}
+	if p.ContentType("x/tags/list") != "application/json" {
+		t.Error("default content type")
+	}
+}
+
+func TestDockerRewriteAndAuth(t *testing.T) {
+	p := &Provider{}
+	if out, err := p.RewriteResponse([]byte("x"), models.Remote{}, "http://p"); out != nil || err != nil {
+		t.Error("docker never rewrites")
+	}
+	h, _ := p.AuthHeaders(context.Background(), models.Remote{Username: "u", Password: "p"})
+	if h.Get("Authorization") == "" {
+		t.Error("expected basic auth header")
+	}
+	h, _ = p.AuthHeaders(context.Background(), models.Remote{})
+	if h.Get("Authorization") != "" {
+		t.Error("no creds, no header")
+	}
+}