Add deb (Debian/apt) local and remote repository support (#111)

Brings Debian/apt to artifactapi with feature parity to the existing rpm support (local + remote), so `.deb` packages can be hosted as a flat apt repo and a Debian/Ubuntu mirror can be cached through the proxy.

- Adds `deb` to the package-type enum and registers a new `internal/provider/deb` provider.
- Classifies `.deb` blobs immutable and the apt index surface (`Packages`, `Release`, `InRelease`, `dists/`, by-hash) mutable so the caching engine revalidates it.
- Parses the `.deb` in pure Go (ar archive to `control.tar.{gz,xz,zst}` to `./control`), storing the raw control stanza plus computed size/md5/sha256 as `deb_metadata`.
- Serves a flat apt repo (`deb [trusted=yes] .../ ./`): generates `Packages`, `Packages.gz` and an unsigned `Release` (returns 404 for `InRelease`/`Release.gpg`), mirroring rpm unsigned repodata / gpgcheck=0 trust model.
- Proxies a remote mirror via `UpstreamURL`/`ContentType`/`AuthHeaders` (HTTP Basic).
- Adds the `deb_metadata` table to `migrate()`, DB access methods, a `MinimalDeb` pure-Go fixture, unit tests, and a `dockere2e` `TestLocalDebRepo`.

---------

Co-authored-by: unkin-agent <unkin-agent@git.unkin.net>
Reviewed-on: #111
Co-authored-by: Unkin Agent <unkin-agent@unkin.net>
Co-committed-by: Unkin Agent <unkin-agent@unkin.net>

e2e-docker/local_test.go

@@ -8,6 +8,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"git.unkin.net/unkin/artifactapi/internal/testsupport"
 )
 
 func uploadFile(t *testing.T, repo, filePath string, body []byte, contentType string) {
@@ -91,3 +93,46 @@ func TestLocalRPMRepodata(t *testing.T) {
 		t.Fatalf("repomd.xml not a valid repodata document: %s", s)
 	}
 }
+
+// TestLocalDebRepo uploads a .deb and validates that the flat apt index
+// (Packages / Release) is generated automatically from the parsed control
+// stanza (the deb-local analog of rpm repodata generation).
+func TestLocalDebRepo(t *testing.T) {
+	createRepo(t, `{"name":"local-deb","package_type":"deb","repo_type":"local"}`)
+	defer deleteRepo(t, "local-deb")
+
+	deb := testsupport.MinimalDeb("e2e-testpkg", "1.0.0", "amd64")
+	uploadFile(t, "local-deb", "e2e-testpkg_1.0.0_amd64.deb", deb, "application/vnd.debian.binary-package")
+
+	// The index is generated asynchronously after upload; poll for it.
+	resp, body := getEventually(t, api("/api/v1/local/local-deb/Packages"), 15*time.Second)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("Packages: status %d: %s", resp.StatusCode, body)
+	}
+	pkgs := string(body)
+	for _, want := range []string{"Package: e2e-testpkg", "Version: 1.0.0", "Architecture: amd64", "Filename: pool/e2e-testpkg_1.0.0_amd64.deb", "SHA256:"} {
+		if !strings.Contains(pkgs, want) {
+			t.Fatalf("Packages missing %q:\n%s", want, pkgs)
+		}
+	}
+
+	resp, body = doRequest(t, http.MethodGet, api("/api/v1/local/local-deb/Release"), nil, "")
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("Release: status %d: %s", resp.StatusCode, body)
+	}
+	rel := string(body)
+	for _, want := range []string{"Architectures: amd64", "SHA256:", "Packages"} {
+		if !strings.Contains(rel, want) {
+			t.Fatalf("Release missing %q:\n%s", want, rel)
+		}
+	}
+
+	// The .deb downloads back byte-identical from its pool path.
+	resp, body = doRequest(t, http.MethodGet, api("/api/v1/local/local-deb/pool/e2e-testpkg_1.0.0_amd64.deb"), nil, "")
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("download deb: status %d: %s", resp.StatusCode, body)
+	}
+	if !bytes.Equal(body, deb) {
+		t.Fatalf("deb content mismatch")
+	}
+}