Add github_deb metadata-only package type (#112)

## Why

This stacks the Debian/apt analog of `github_rpm` on top of the deb local+remote work (#111). It lets a GitHub repo's `.deb` release assets be consumed as a real apt repository without artifactapi ever precaching whole packages: it derives per-asset control metadata from a ranged prefix fetch, synthesizes a flat apt repo from the cache, and redirects the actual `.deb` downloads to a backend `releases_remote` (the generic github.com remote).

Base is `benvin/deb-local-remote` (stacked) to keep the diff atomic.

## How

- Adds `github_deb` to the package-type enum and validity map.
- Adds the `github_deb` provider mirroring `github_rpm`: `ServeRemote` serves `Packages`/`Packages.gz`/`Release`, returns 404 for `InRelease`/`Release.gpg` (unsigned, consumed via `[trusted=yes]`), and 302-redirects `*.deb` to `{proxyBaseURL}/api/v1/remote/{releases_remote}/{path}`; cold-start prime with a retryable 503.
- `deriveAsset` ranged-GETs the front of the `.deb` (an `ar` archive), locates and fully reads `control.tar.*`, and parses the control paragraph — doubling the range if the control member is truncated. The Packages `SHA256` comes from the GitHub asset `digest` when present, else a one-time full stream; `MD5sum` is left unset (apt verifies against SHA256 under `[trusted=yes]`).
- Adds a `github_deb` background Syncer (own worker pool, shared rate limiter, deduped queue) with per-remote DB-lease-gated scans so only one replica scans per window.
- Adds the `github_deb_sync_state` table plus `ListGitHubDebRemotes` / `ClaimGitHubDebSyncLease` / `ReleaseGitHubDebSyncLease` DB helpers, kept separate from the rpm ones.
- Primes `github_deb` remotes on create and runs the deb syncer alongside the rpm one; prime-on-create is routed by package type.
- Reuses the deb apt-index generators and control parser; the Packages generator now skips empty hash lines so a SHA256-only entry is valid.

## Notes / deviations

- **Filename convention:** the `Filename` stored in the Packages index is the **github-relative** asset path (same as rpm's `assetPath`), not `pool/<asset>`. This is required for the `.deb` 302 to `{releases_remote=github}/{path}` to resolve against github.com; it still matches the `*.deb` redirect rule.
- **GitHub client helpers** (releases pagination, ranged GET, auth headers) are duplicated into the deb package rather than shared, because the rpm equivalents are unexported in `package rpm` and the task requires not modifying the rpm provider.
- `go build`, `go vet`, `go mod tidy`, and `make test` (`-race`, incl. the Postgres lease integration tests) all pass; pre-commit clean.

Do not merge — for review.

---------

Co-authored-by: unkin-agent <unkin-agent@git.unkin.net>
Reviewed-on: #112
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/server/server.go

@@ -21,7 +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/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"
@@ -35,6 +35,7 @@ import (
 	"git.unkin.net/unkin/artifactapi/internal/storage"
 	"git.unkin.net/unkin/artifactapi/internal/tfsign"
 	"git.unkin.net/unkin/artifactapi/internal/virtual"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
 type Server struct {
@@ -50,6 +51,7 @@ type Server struct {
 	tfRegistry   *tfregistry.Handler
 	gc           *gc.Collector
 	syncer       *rpm.Syncer
+	debSyncer    *deb.Syncer
 }
 
 func New(cfg *config.Config, version string) (*Server, error) {
@@ -97,6 +99,12 @@ func New(cfg *config.Config, version string) (*Server, error) {
 		Workers:      cfg.GitHubSyncWorkers,
 		PollInterval: time.Duration(cfg.GitHubSyncPollInterval) * time.Second,
 	})
+	debSyncer := deb.NewSyncer(db, deb.SyncConfig{
+		RatePerSec:   cfg.GitHubSyncRatePerSec,
+		Burst:        cfg.GitHubSyncBurst,
+		Workers:      cfg.GitHubSyncWorkers,
+		PollInterval: time.Duration(cfg.GitHubSyncPollInterval) * time.Second,
+	})
 
 	// The terraform registry signs with a GPG key. A configured file wins (BYO
 	// key); otherwise artifactapi generates one on first start and persists it in
@@ -129,6 +137,7 @@ func New(cfg *config.Config, version string) (*Server, error) {
 		tfRegistry:   tfRegistry,
 		gc:           collector,
 		syncer:       syncer,
+		debSyncer:    debSyncer,
 	}
 
 	s.router = s.routes()
@@ -158,7 +167,10 @@ func (s *Server) routes() chi.Router {
 	r.Mount("/api/v1", proxyHandler.Routes())
 	r.Mount("/v2", proxyHandler.DockerV2Routes())
 
-	remotesHandler := v2.NewRemotesHandler(s.db, s.syncer)
+	remotesHandler := v2.NewRemotesHandler(s.db, map[models.PackageType]v2.Primer{
+		models.PackageGitHubRPM: s.syncer,
+		models.PackageGitHubDeb: s.debSyncer,
+	})
 	virtualsHandler := v2.NewVirtualsHandler(s.db)
 	healthHandler := v2.NewHealthHandler(s.db, s.cache, s.store)
 	statsHandler := v2.NewStatsHandler(s.db)
@@ -226,6 +238,7 @@ func (s *Server) newHTTPServer() *http.Server {
 func (s *Server) Run(ctx context.Context) error {
 	go s.gc.Run(ctx)
 	go s.syncer.Run(ctx)
+	go s.debSyncer.Run(ctx)
 
 	httpServer := s.newHTTPServer()
 
@@ -247,6 +260,7 @@ func (s *Server) Run(ctx context.Context) error {
 func (s *Server) RunOnListener(ctx context.Context, ln net.Listener) error {
 	go s.gc.Run(ctx)
 	go s.syncer.Run(ctx)
+	go s.debSyncer.Run(ctx)
 
 	httpServer := s.newHTTPServer()