feat: background syncer for github_rpm remotes

Lazy per-replica scans re-derived RPM metadata on the client request path
and, run independently on every replica, multiplied GitHub queries by the
replica count. A single background syncer with a shared rate limit, ETag
conditional checks, and a DB lease keeps metadata fresh off the request path
while bounding GitHub load to ~once per mutable_ttl across the fleet.

- Add a single per-process syncer (started at boot, stopped on shutdown) that
  owns a deduped/coalescing work queue, a worker pool, and one global
  token-bucket rate limiter bound onto the github provider so every GitHub call
  (releases list + each ranged asset GET) acquires a token first.
- Check each github_rpm remote for new/changed releases on its mutable_ttl
  cadence; derive only new/changed assets incrementally and prune assets that
  disappear upstream, so repodata is served from primed DB rows.
- Prime metadata in the background on remote creation; the create call never
  blocks on a derive.
- Send the stored releases-list ETag as If-None-Match; a 304 derives nothing
  (and does not count against GitHub's rate limit), making an unchanged repo
  nearly free.
- Coordinate replicas through a github_rpm_sync_state row (last_synced_at,
  etag, sync_lease_owner, sync_lease_expires): a periodic scan runs only for
  the replica that atomically claims the lease, bounding total GitHub load to
  ~once per mutable_ttl regardless of replica count.
- Keep the request path fast: serve current cache, enqueue a prime on an empty
  cache, and return a bounded wait then a retryable 503 rather than blocking on
  a cold derive.
- Add GITHUB_SYNC_RATE/BURST/WORKERS/POLL_INTERVAL config (conservative
  defaults) and document the syncer in the README.

internal/server/server.go

@@ -27,7 +27,7 @@ import (
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/npm"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/puppet"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/pypi"
-	_ "git.unkin.net/unkin/artifactapi/internal/provider/rpm"
+	"git.unkin.net/unkin/artifactapi/internal/provider/rpm"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/terraform"
 	"git.unkin.net/unkin/artifactapi/internal/proxy"
 	"git.unkin.net/unkin/artifactapi/internal/storage"
@@ -47,6 +47,7 @@ type Server struct {
 	localHandler *v2.LocalHandler
 	tfRegistry   *tfregistry.Handler
 	gc           *gc.Collector
+	syncer       *rpm.Syncer
 }
 
 func New(cfg *config.Config, version string) (*Server, error) {
@@ -69,6 +70,12 @@ func New(cfg *config.Config, version string) (*Server, error) {
 	localHandler := v2.NewLocalHandler(db, s3)
 	virtEngine := virtual.NewEngine(db, engine)
 	collector := gc.New(db, s3, 1*time.Hour)
+	syncer := rpm.NewSyncer(db, rpm.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
@@ -100,6 +107,7 @@ func New(cfg *config.Config, version string) (*Server, error) {
 		localHandler: localHandler,
 		tfRegistry:   tfRegistry,
 		gc:           collector,
+		syncer:       syncer,
 	}
 
 	s.router = s.routes()
@@ -129,7 +137,7 @@ func (s *Server) routes() chi.Router {
 	r.Mount("/api/v1", proxyHandler.Routes())
 	r.Mount("/v2", proxyHandler.DockerV2Routes())
 
-	remotesHandler := v2.NewRemotesHandler(s.db)
+	remotesHandler := v2.NewRemotesHandler(s.db, s.syncer)
 	virtualsHandler := v2.NewVirtualsHandler(s.db)
 	healthHandler := v2.NewHealthHandler(s.db, s.cache, s.store)
 	statsHandler := v2.NewStatsHandler(s.db)
@@ -196,6 +204,7 @@ func (s *Server) newHTTPServer() *http.Server {
 
 func (s *Server) Run(ctx context.Context) error {
 	go s.gc.Run(ctx)
+	go s.syncer.Run(ctx)
 
 	httpServer := s.newHTTPServer()
 
@@ -216,6 +225,7 @@ 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)
 
 	httpServer := s.newHTTPServer()