Retire in-code comments that outlived what they described (B-69)

- app/tx/reconcile.py called the payout retry "a future payout-retry
  routine — still an open gap". It shipped as B-26: clearing payout_txid
  leaves the round in exactly the state _retry_payout_if_due picks up, so
  an abandoned payout rebuilds itself and the log line next to it is an
  alert, not the recovery path. Reading it the old way, an operator would
  go hand-fix a round the scheduler was already retrying.
- app/db/base.py sized the SQLite busy timeout against "five concurrent
  background tasks" and then listed only the non-listener ones; the
  lifespan starts six.
- The third item (app/auth/routes.py citing B-31 where it meant B-33) was
  already correct in the tree; the test pins it so it stays that way.

tests/unit/test_code_comments.py derives the task count from the lifespan's
own create_task calls rather than restating it, so the comment fails the
next time a task is added or removed instead of quietly going stale again.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -61,7 +61,7 @@ python -m pytest                    # all tests
 python -m pytest tests/unit/test_hd.py   # one file
 ```
 
-345 unit tests cover HD derivation, PSBT building, the Electrum client, bets,
+349 unit tests cover HD derivation, PSBT building, the Electrum client, bets,
 deposits, withdrawals, the round/draw engine, RBF fee-bumping, admin config,
 the pending-inclusive balance calculation, and the SSE push channel. No
 automated integration tests against a live Electrum connection — mainnet