Verify the master key against the database at startup (B-89, SEC-49, SEC-50, SEC-09)

The lifespan decrypts the key and compares a fixed sample (reserve/round pool
addresses, the 10 lowest and highest user addresses, unrevealed round secrets)
with the database, and refuses to serve on any mismatch without printing key
material. The key scripts refuse a new or imported key that would not reproduce
the stored addresses, and write 0600.

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

README.md

@@ -62,7 +62,7 @@ python -m pytest                    # all tests
 python -m pytest tests/unit/test_hd.py   # one file
 ```
 
-702 unit tests cover HD derivation, PSBT building, the Electrum client, bets,
+711 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