feat(scheduler,oauth-setup): Phase 3F — Scheduler + OAuth bootstrap helper

[smallchungus]

Phase 3F closes Phase 3.

• internal/scheduler — Scheduler.PollOnce iterates users; on first sync seeds cursor from Users.GetProfile (forward-sync only, no backfill); on subsequent polls calls gmail.LatestMessageIDs and enqueues fetch jobs.
• New store methods: ListUsers, GetGmailSyncState, SetGmailSyncState (UPSERT).
• cmd/scheduler — 5 min poll loop, signal-handled graceful shutdown.
• cmd/oauth-setup — interactive helper that walks the Google OAuth dance once, listens on http://localhost:8888/callback, exchanges the code, encrypts and saves the token. Auto-creates the user row if needed.
• README: Real Gmail->Drive setup walkthrough.

Phase 3 milestone: the full Gmail -> PDF -> Drive pipeline runs end-to-end with a real Google account.