Add per-remote sync_interval and spread GitHub syncs (#166)

github_* remotes sharing a mutable_ttl all become due at the same moment, so many remotes created or synced together burst GitHub at once.

- add per-remote `sync_interval` (seconds) for github_rpm/github_deb/github_alpine remotes
- offset each remote's slots by a stable name-hashed phase so remotes on one interval spread out
- compute slots and the first-sync gate in the lease claim from the DB clock so replicas agree
- wait for the first slot on a scheduled first sync and skip the create-time prime; cold-cache primes stay immediate
- keep the mutable_ttl freshness predicate unchanged when `sync_interval` is unset
- document `sync_interval` in README, rpm docs and UI types

Reviewed-on: #166
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

migrations/0009_remote_sync_interval.sql

@@ -0,0 +1 @@
+ALTER TABLE remotes ADD COLUMN IF NOT EXISTS sync_interval INTEGER NOT NULL DEFAULT 0;