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>

README.md

@@ -46,12 +46,43 @@ metadata comes from a **ranged GET of just the RPM header** (the header sits at
 the front of the file, so the whole package is never downloaded); the sha256
 checksum comes from the GitHub asset `digest` when present, else a one-time
 lazy stream. Derived metadata is cached (keyed by asset) so repodata generation
-is cheap on repeat, and refreshed no more often than `mutable_ttl`.
+is served from primed DB rows, never a cold on-demand derive.
 
 Each package's `<location>` points back at the remote, which **302-redirects**
 the download to the `releases_remote` — an existing generic `github.com` remote
 that streams the actual bytes. `dnf` follows the redirect transparently.
 
+#### Background syncer
+
+A single process-wide **background syncer** keeps every `github_rpm` remote's
+derived metadata current off the client request path:
+
+- **Prime on create.** Creating a `github_rpm` remote enqueues a background prime
+  scan, so its metadata is derived right away without blocking the create call.
+  The first `dnf` request is served from cache. If a request arrives before the
+  prime lands, it returns a retryable `503` (with `Retry-After`) rather than
+  serving an empty repo or blocking on a multi-minute derive.
+- **Periodic re-check, driven by `mutable_ttl`.** Each remote is re-checked for
+  new or changed releases no more often than its `mutable_ttl`. New/changed
+  assets are derived incrementally; assets already cached are never re-fetched,
+  and assets that disappear upstream are pruned.
+- **ETag / 304 conditional requests.** The releases-list `ETag` is stored per
+  remote and sent as `If-None-Match`; a `304 Not Modified` means nothing changed
+  and the syncer derives nothing. GitHub does not count `304` conditional
+  responses against the rate limit, so an unchanged repo is nearly free — this is
+  the main lever keeping GitHub traffic low.
+- **Global rate limit.** Every GitHub call (releases list + each ranged asset
+  header GET) passes through a single token-bucket limiter **shared across all
+  remotes**, so GitHub is never hammered. Configure a token (`password`) on the
+  remote for the higher authenticated rate limit (~5000/hr vs ~60/hr
+  unauthenticated).
+- **Multi-replica coordination.** State is shared through the database. Before a
+  periodic scan a replica must atomically claim a per-remote lease
+  (`github_rpm_sync_state`: `last_synced_at`, `etag`, `sync_lease_owner`,
+  `sync_lease_expires`); only the winner scans. This bounds total GitHub load to
+  ~once per `mutable_ttl` regardless of replica count, and the shared `etag`
+  lets any replica issue the conditional request.
+
 ```hcl
 # Backend that serves the actual .rpm bytes from github.com.
 resource "artifactapi_remote_generic" "github" {
@@ -242,6 +273,10 @@ S3 client supports MinIO, Ceph RGW, and AWS S3 (via minio-go).
 | `MINIO_BUCKET` | `artifacts` | S3 bucket |
 | `MINIO_SECURE` | `false` | Use HTTPS for S3 |
 | `MINIO_REGION` | | S3 region (AWS) |
+| `GITHUB_SYNC_RATE` | `1` | `github_rpm` syncer global GitHub request rate (req/s), shared across all remotes. `1`/s = 3600/hr, under an authenticated token's ~5000/hr; unauthenticated (~60/hr) relies on ETag/304 |
+| `GITHUB_SYNC_BURST` | `5` | Token-bucket burst for the shared limiter |
+| `GITHUB_SYNC_WORKERS` | `3` | Concurrent `github_rpm` scan workers |
+| `GITHUB_SYNC_POLL_INTERVAL` | `60` | Base scheduler tick in seconds; per-remote cadence is its `mutable_ttl`, enforced by the DB lease |
 
 ## Development