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/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.