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>

pkg/models/package_type.go

@@ -11,6 +11,7 @@ const (
 	PackagePyPI      PackageType = "pypi"
 	PackageNPM       PackageType = "npm"
 	PackageRPM       PackageType = "rpm"
+	PackageDeb       PackageType = "deb"
 	PackageAlpine    PackageType = "alpine"
 	PackagePuppet    PackageType = "puppet"
 	PackageTerraform PackageType = "terraform"
@@ -25,6 +26,7 @@ var validPackageTypes = map[PackageType]bool{
 	PackagePyPI:      true,
 	PackageNPM:       true,
 	PackageRPM:       true,
+	PackageDeb:       true,
 	PackageAlpine:    true,
 	PackagePuppet:    true,
 	PackageTerraform: true,