refactor!: collapse state column + release_stuck into lease-only delivery

[lesnik512]

The state column duplicated information already carried by acquired_token: state='processing' was equivalent to acquired_token IS NOT NULL. The redundancy required a third async loop (_release_stuck_loop) to flip stuck rows back to pending plus a Postgres advisory lock to coordinate that loop across replicas.

Replace both with a single lease check inside the fetch CTE: a row is available iff its lease is unset or expired (acquired_at < now() - lease_ttl_seconds). This deletes ~50 lines, drops one async task per subscriber, and shrinks worst-case crash recovery from ~7.5 min to ~60s with the new default TTL.

Breaking changes:

• OutboxState enum removed from public exports.
• state column dropped from make_outbox_table.
• OutboxClient.release_stuck removed.
• release_stuck_timeout + release_stuck_interval kwargs replaced with single lease_ttl_seconds: float = 60.0 (was release_stuck_timeout=300.0) on subscriber(), OutboxRoute, and create_subscriber.
• OutboxClient.fetch requires new lease_ttl_seconds kwarg.

Migration:

ALTER TABLE outbox DROP CONSTRAINT outbox_state_check;
ALTER TABLE outbox DROP COLUMN state;
DROP INDEX IF EXISTS outbox_pending_idx;
CREATE INDEX outbox_pending_idx ON outbox (queue, next_attempt_at)
  WHERE acquired_token IS NULL;

103 tests pass with 100% coverage.