feat(web): review queue — inbox + policy-merged digest (Phase 4)

The backstop for work no link reached. /inbox lists every open proposal
on the instance ("waiting on you") and, below it, the digest of recently
policy-merged content — the firehose a human sees after the fact, which
is the whole reason approval=policy is kept distinct from human.

- service.InboxProposals / DigestProposals list instance-wide (one
  reviewer, so a per-space inbox would make them hunt), mapping each
  stored proposal's space_id back to a reference once from the space list.
- web/inbox.go + inbox.html render the two sections; the landing page
  links the queue for a logged-in owner.

Follow-up: the digest currently shows recent policy-merges rather than
"since you last looked" — the digest_mark table exists to track that, but
advancing it is a write and GET stays pure. Filed separately.