feat: background syncer for github_rpm remotes (#108)

## Why

Builds on #107 (merged), which derives `github_rpm` RPM metadata lazily on the
client request path, single-flighted per replica. Two problems remain: the
derive still happens per replica, so across a multi-replica deployment the same
releases are scanned and re-derived N times, multiplying GitHub queries; and a
cold cache blocks the first request on a full derive. GitHub's rate limits are
low (~60/hr unauthenticated, ~5000/hr authenticated), so this needs a single
coordinated syncer with a shared rate limit and conditional requests.

## How

- Add a single per-process background syncer (started at boot, cleanly stopped
  on shutdown) that owns a deduped/coalescing work queue, a worker pool, and one
  global token-bucket rate limiter (`golang.org/x/time/rate`) bound onto the
  github provider so every GitHub call (releases list + each ranged asset GET)
  acquires a token first.
- Re-check each `github_rpm` remote for new/changed releases on its existing
  `mutable_ttl` cadence; derive only new/changed assets incrementally and prune
  assets that disappear upstream. Repodata is served from primed DB rows.
- Prime metadata in the background on remote creation; the create call returns
  immediately.
- Send the stored releases-list `ETag` as `If-None-Match`; a `304` derives
  nothing and is not counted against GitHub's rate limit, so an unchanged repo
  is 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; the ETag is shared
  through the same row.
- 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` / `GITHUB_SYNC_BURST` / `GITHUB_SYNC_WORKERS` /
  `GITHUB_SYNC_POLL_INTERVAL` config with conservative defaults (1 req/s, burst
  5, 3 workers, 60s tick) and document the syncer in the README.

## Tests

- Unit (httptest, Range/ETag-aware fixture): `304` releases response derives
  nothing; incremental derive fetches only the newly added asset; the shared
  limiter caps request rate; work-queue enqueues coalesce to one job; prime
  enqueues a job; a held lease stops a second replica from scanning; cold-start
  serves `503` while warm cache serves `200`.
- DB integration (testcontainers postgres): the real lease SQL — one holder at a
  time, recency gate blocks a too-soon periodic re-claim, prime (freshness 0)
  bypasses recency but respects a live lease.
- Docker e2e re-run: `dnf install dotvault` works; prime-on-create derives in the
  background at ~1 req/s (global limiter); `dnf makecache` served fast from the
  priming cache (no cold block); clean shutdown mid-scan, no panics.

## Notes

- Reuses `mutable_ttl` as the check interval (no new per-remote field), per brief.

Reviewed-on: #108
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/provider/rpm/github_test.go

@@ -66,11 +66,14 @@ func (f *fakeStore) ListRPMMetadataEntries(ctx context.Context, _ string) ([]pro
 // Range support) for a set of packages. digest controls whether the asset
 // carries a sha256 digest (no-download path) or not (compute path).
 type githubFixture struct {
-	srv      *httptest.Server
-	rpmBytes map[string][]byte // asset filename -> bytes
-	rangeHit map[string]int    // asset filename -> number of ranged GETs
-	fullHit  map[string]int    // asset filename -> number of full GETs
-	mu       sync.Mutex
+	srv         *httptest.Server
+	rpmBytes    map[string][]byte // asset filename -> bytes
+	rangeHit    map[string]int    // asset filename -> number of ranged GETs
+	fullHit     map[string]int    // asset filename -> number of full GETs
+	etag        string            // when set, served as ETag; matching If-None-Match yields 304
+	releasesHit int               // total releases-list requests (200 + 304)
+	notModHit   int               // releases-list requests answered 304
+	mu          sync.Mutex
 }
 
 func newGitHubFixture(t *testing.T, withDigest bool) *githubFixture {
@@ -89,6 +92,19 @@ func newGitHubFixture(t *testing.T, withDigest bool) *githubFixture {
 			w.Write([]byte("[]"))
 			return
 		}
+		f.mu.Lock()
+		f.releasesHit++
+		etag := f.etag
+		if etag != "" && r.Header.Get("If-None-Match") == etag {
+			f.notModHit++
+			f.mu.Unlock()
+			w.WriteHeader(http.StatusNotModified)
+			return
+		}
+		f.mu.Unlock()
+		if etag != "" {
+			w.Header().Set("ETag", etag)
+		}
 		var assets []map[string]any
 		for name := range f.rpmBytes {
 			a := map[string]any{