Document sync_interval for deb/alpine and fix created_at gate on existing rows (#176)

`sync_interval` is documented only for github_rpm, and the claim gate holding back remotes created inside the current slot also blocks remotes that already have a sync state row: the gate filters the INSERT SELECT, so ON CONFLICT never runs and a failed cold-cache prime waits a full slot instead of its backoff.

- document `sync_interval` for github_deb/github_alpine and in the caching field table
- apply the created_at gate only when no sync state row exists
- test 0→N and N→0 interval changes and the insert-only gate against Postgres
- test that a negative `sync_interval` on update returns 400

Reviewed-on: #176
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/deb/remote-github.md

@@ -5,3 +5,16 @@ Shared classification, TTL and eviction rules: [caching](../common/caching.md).
 | Type | Mutable (auto-detected) | Immutable (auto-detected) |
 |---|---|---|
 | `github_deb` | `Packages*`, `Release*` (synthesized) | `.deb` (redirected) |
+
+## Release sync
+
+A background syncer re-scans the GitHub releases list per remote, sharing one
+lease per remote across replicas through `github_deb_sync_state`.
+
+- **`sync_interval`** (seconds) re-checks once per interval at a stable
+  per-remote offset, so remotes on the same interval spread out instead of
+  syncing together; its first scheduled sync waits for that slot (a cold-cache
+  request still primes at once). Unset, it re-checks no more often than its
+  `mutable_ttl`.
+- A failed scan keeps serving the last good index and retries with exponential
+  backoff.