Confine phased sync slots to sync_interval remotes

Unset remotes keep the mutable_ttl freshness predicate. Scheduled remotes
compute their slot in Go, skip the create-time prime and wait for their
first slot; a cold-cache prime still runs at once. Document sync_interval.

ui/src/api/types.ts

@@ -8,6 +8,7 @@ export interface Remote {
   immutable_ttl: number;
   mutable_ttl: number;
   check_mutable: boolean;
+  sync_interval?: number;
   patterns: string[];
   blocklist: string[];
   mutable_patterns: string[];