Document sync_interval for deb/alpine and test interval changes

- add sync_interval to the github_deb/github_alpine docs and the caching field table
- apply the created_at slot gate only when no sync state row exists
- test 0<->N interval changes, the insert-only gate and a negative sync_interval on update

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.