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/keystore.py

@@ -1,3 +1,5 @@
+import os
+
 from cryptography.fernet import Fernet
 
 from app.config import settings
@@ -20,3 +22,12 @@ def encrypt_round_seed(server_seed_hex: str) -> str:
 
 def decrypt_round_seed(token: str) -> str:
     return Fernet(settings.xprv_encryption_key).decrypt(token.encode()).decode()
+
+
+def write_secret_file(path: str, data: bytes) -> None:
+    """B-89 (SEC-50): created `0600` from the start (never through a wider mode) and
+    rewritten with the same mode, so a key file is not readable by other users."""
+    fd = os.open(path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+    with os.fdopen(fd, "wb") as f:
+        os.fchmod(f.fileno(), 0o600)
+        f.write(data)