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/rpm/remote-github.md

@@ -37,9 +37,9 @@ derived metadata current off the client request path:
   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`. New/changed
-  assets are derived incrementally; assets already cached are never re-fetched,
-  and assets that disappear upstream are pruned.
+  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
@@ -54,8 +54,8 @@ derived metadata current off the client request path:
   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 `sync_interval` (else `mutable_ttl`) regardless of replica count, and the shared `etag`
-  lets any replica issue the conditional request.
+  ~once per `sync_interval` (else `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.