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>

tests/unit/conftest.py

@@ -0,0 +1,25 @@
+"""Every test runs against a throwaway master key, never the operator's real one.
+
+B-80: opening a round now derives its pool address, so code paths that merely open a
+round (the scheduler, `POST /bets`, the round route) need a master key — and without
+this they silently read whatever `MASTER_KEY_PATH`/`XPRV_ENCRYPTION_KEY` of the local
+`.env` point at, i.e. the real encrypted key on a developer machine and nothing at all
+on a clean checkout. Tests that build their own key (most of them) simply override it.
+"""
+
+import pytest
+from cryptography.fernet import Fernet
+
+from app.config import settings
+
+
+@pytest.fixture(autouse=True)
+def _throwaway_master_key(tmp_path, monkeypatch):
+    monkeypatch.setattr(settings, "master_key_path", str(tmp_path / "autouse-master.xprv.enc"))
+    monkeypatch.setattr(settings, "xprv_encryption_key", Fernet.generate_key().decode())
+    from app.wallet import hd
+
+    hd._account_key = None
+    hd.generate_master_key()
+    yield
+    hd._account_key = None