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>

app/wallet/hd.py

@@ -5,7 +5,7 @@ from embit.bip32 import HDKey
 from embit.ec import PrivateKey
 
 from app.config import settings
-from app.wallet.keystore import decrypt_xprv, encrypt_xprv
+from app.wallet.keystore import decrypt_xprv, encrypt_xprv, write_secret_file
 from app.wallet.plm_network import ACCOUNT_PATH, PLM_MAINNET
 
 _account_key: HDKey | None = None
@@ -17,8 +17,7 @@ def generate_master_key(overwrite: bool = False) -> None:
     if os.path.exists(settings.master_key_path) and not overwrite:
         raise FileExistsError(f"{settings.master_key_path} already exists")
     root = HDKey.from_seed(os.urandom(32), version=PLM_MAINNET["xprv"])
-    with open(settings.master_key_path, "wb") as f:
-        f.write(encrypt_xprv(root.to_base58(version=PLM_MAINNET["xprv"])))
+    write_secret_file(settings.master_key_path, encrypt_xprv(root.to_base58(version=PLM_MAINNET["xprv"])))
 
 
 def _load_account_key() -> HDKey: