Give every round its own pool address (B-80, RND-35, DAT-60, DAT-11, REG-14, SEC-08, BET-12, RND-22, WDR-14, SEC-43, API-23, API-36)

All bets of all rounds paid one address (m/84'/746'/0'/1/0), so its history
grew forever, other rounds' money and stray coins mixed with a round's own, and
the payout picked from everything it held.

- hd.py: derive_pool_key/derive_pool_address(round_id) on branch 2
  (m/84'/746'/0'/2/<round_id>); no argument is the reserve address 1/0.
  derive_round_pool_key checks the key against the address stored on the round,
  so a payout can never be signed for money its key does not control.
- rounds.pool_address (migration 477bddae1a84, existing rounds backfilled with
  the reserve address since that is where their bets went; add_column, not a
  batch alter, so the expression indexes stay): written in the transaction that
  inserts the round, once its id exists.
- place_bet pays round.pool_address; the payout reads, signs and takes change at
  the round's own address; own_address_for and bump_fee's _signing_context follow
  the round (confirmation poller and reconciler pass round_id).
- Withdrawals to any round pool address, the reserve address or fee_address are
  refused (withdrawal_to_platform_address, 400, all seven languages).
- GET /rounds/current and GET /admin/rounds expose pool_address.
- RND-22/DAT-11 stay "da implementare" for B-82: the payout still spends every
  confirmed coin at the round's address, not only the participants' outpoints
  (participants carry no bet_vout yet).
- tests/unit/conftest.py: every test now gets a throwaway master key. Opening a
  round derives an address, and tests that merely opened a round were silently
  reading the real key from the local .env.
- Existing tests changed because the spec changed: payout fixtures give their
  round a pool_address; the reconciler's address-cache test uses two payouts of
  one round (rows of different rounds no longer share an address);
  own_address_for now takes the round id.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/adr/ADR-03-ogni-round-ha-proprio-indirizzo-montepremi.md

@@ -1,6 +1,6 @@
 # ADR-03 — Ogni round ha il proprio indirizzo del montepremi
 
-- **Stato:** Accettata — implementazione da completare (B-80)
+- **Stato:** Accettata
 - **Area:** Custodia, chiavi e indirizzi
 - **Fonte di verità per:** la scelta di progetto descritta qui sotto; i requisiti (`PREFISSO-NN`) che la attuano stanno nei documenti di dominio, indicati nelle *Conseguenze*.
 
@@ -30,10 +30,7 @@ commissioni è rifiutato. Il ramo `0` resta riservato agli utenti.
   lì, visibile ma fuori da ogni pagamento; il margine fra partecipanti e
   ingressi del pagamento ([ADR-21](ADR-21-tre-limiti-distinti-sul-numero-monete.md)) non si consuma più nel tempo ([LIM-17]).
   Regole in [ciclo-di-vita-del-round.md](../ciclo-di-vita-del-round.md#4--pagamento)
-  e [scommesse-e-prelievi.md](../scommesse-e-prelievi.md#prelievo) (B-80).
+  e [scommesse-e-prelievi.md](../scommesse-e-prelievi.md#prelievo).
 
-*Codice:* `app/wallet/hd.py:derive_pool_key` · `app/wallet/hd.py:derive_pool_address` · `app/rounds/service.py:open_new_round_if_needed` ·
-*Test:* `tests/unit/test_pending_address.py`
-*Stato:* da implementare (B-80) · *Oggi:* un unico indirizzo del montepremi
-(`1/0`) riceve le scommesse di tutti i round e il pagamento sceglie fra
-tutte le sue monete.
+*Codice:* `app/wallet/hd.py:derive_pool_key` · `app/wallet/hd.py:derive_pool_address` · `app/wallet/hd.py:derive_round_pool_key` · `app/rounds/service.py:open_new_round_if_needed` ·
+*Test:* `tests/unit/test_round_pool_address.py`, `tests/unit/test_pending_address.py`