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>

internal/server/server.go

@@ -21,6 +21,7 @@ import (
 	"git.unkin.net/unkin/artifactapi/internal/gc"
 	"git.unkin.net/unkin/artifactapi/internal/githubauth"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/alpine"
+	_ "git.unkin.net/unkin/artifactapi/internal/provider/deb"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/docker"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/generic"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/goproxy"