Add a periodic deposit reconciler, and stop losing subscribe tasks (B-30)

Deposits were credited exclusively by scripthash-change notifications,
with nothing re-verifying a user's balance against the chain if a
subscription was ever silently lost. address_for_new_user's subscribe
was fire-and-forget: the task wasn't retained, so it could be
garbage-collected mid-flight, and any failure (including self.client
turning None between the check and the task running) vanished into
asyncio's default unretrieved-exception handler instead of being
logged anywhere. On an otherwise healthy, long-lived connection there
may be no reconnect for days to re-subscribe everyone, so a user in
that state never saw their deposits.

address_for_new_user now retains the task and logs its exception if
it fails. New app/deposits/reconcile.py adds DepositReconciler, a
periodic sweep (every 5 minutes, gated on the Electrum client being
connected, same shape as tx/reconcile.py) that round-robins over every
user and calls the listener's own refresh_user (renamed from
_refresh_user since it's now called from outside the class) - so the
notification-driven and periodic paths can never behave differently.
Deliberately sweeps every user rather than only ones missing from the
in-memory scripthash map, since that map can't tell "never subscribed"
apart from "subscribed, but the server stopped delivering
notifications for it". Wired into app/main.py's lifespan alongside the
other three background reconcilers.

Suite grows from 176 to 182 tests. BUGS.md moves B-30 to Previously
fixed.

app/deposits/service.py

@@ -113,7 +113,7 @@ async def find_utxos_missing_from(session: AsyncSession, user_id: int, entries:
     Returning a row here is *not* proof it was actually spent — only that this one
     server's reply no longer lists it. A single broken, behind, or malicious
     server could otherwise zero a user's balance on one bad reply, which is why
-    the caller (electrum/listener.py:_refresh_user) must independently
+    the caller (electrum/listener.py:refresh_user) must independently
     corroborate each candidate against other configured servers before treating
     it as genuine, rather than this function marking anything itself.