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/database/postgres.go

@@ -164,6 +164,24 @@ func (db *DB) migrate() error {
 		ALTER TABLE rpm_metadata ADD COLUMN IF NOT EXISTS conflicts JSONB DEFAULT '[]';
 		ALTER TABLE rpm_metadata ADD COLUMN IF NOT EXISTS obsoletes JSONB DEFAULT '[]';
 
+		CREATE TABLE IF NOT EXISTS deb_metadata (
+			id              BIGSERIAL PRIMARY KEY,
+			repo_name       TEXT NOT NULL,
+			file_path       TEXT NOT NULL,
+			content_hash    TEXT NOT NULL,
+			name            TEXT NOT NULL,
+			version         TEXT NOT NULL,
+			architecture    TEXT NOT NULL,
+			control         TEXT NOT NULL,
+			size            BIGINT DEFAULT 0,
+			md5             TEXT DEFAULT '',
+			sha256          TEXT DEFAULT '',
+			created_at      TIMESTAMPTZ DEFAULT NOW(),
+			UNIQUE(repo_name, file_path)
+		);
+
+		CREATE INDEX IF NOT EXISTS idx_deb_metadata_repo ON deb_metadata(repo_name);
+
 		CREATE TABLE IF NOT EXISTS github_rpm_sync_state (
 			remote_name        TEXT PRIMARY KEY,
 			etag               TEXT DEFAULT '',