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/provider/provider.go

@@ -61,6 +61,7 @@ type PostDeleteHook interface {
 
 type MetadataStore interface {
 	InsertRPMMetadata(ctx context.Context, meta *RPMMetadata) error
+	InsertDebMetadata(ctx context.Context, meta *DebMetadata) error
 }
 
 // RemoteServer lets a remote provider fully answer a request itself instead of
@@ -83,12 +84,37 @@ type RemoteMetadataStore interface {
 
 type MetadataDeleter interface {
 	DeleteRPMMetadata(ctx context.Context, repoName, filePath string) error
+	DeleteDebMetadata(ctx context.Context, repoName, filePath string) error
 }
 
 type RPMMetadataReader interface {
 	ListRPMMetadataEntries(ctx context.Context, repoName string) ([]RPMMetadata, error)
 }
 
+// DebMetadataReader is the read surface the deb LocalIndexer needs to
+// regenerate a flat apt repository (Packages/Release) from stored rows.
+// *database.DB satisfies it; ServeLocalIndex type-asserts the FileStore to it,
+// mirroring how the rpm provider reaches its RPMMetadataReader.
+type DebMetadataReader interface {
+	ListDebMetadataEntries(ctx context.Context, repoName string) ([]DebMetadata, error)
+}
+
+// DebMetadata is the derived per-package metadata for a Debian .deb, carrying
+// the full raw control stanza so the Packages index can be regenerated
+// faithfully alongside the computed size/md5/sha256 apt requires.
+type DebMetadata struct {
+	RepoName     string
+	FilePath     string
+	ContentHash  string
+	Name         string
+	Version      string
+	Architecture string
+	Control      string
+	Size         int64
+	MD5          string
+	SHA256       string
+}
+
 type RPMMetadata struct {
 	RepoName      string
 	FilePath      string