Wire up the FastAPI app and master-key bootstrap script

app/main.py assembles the lifespan-managed background tasks (Electrum
listener, round scheduler, confirmation poller, RBF bumper) and mounts
all routers behind a /health check. generate_master_key.py is the
one-time ops script that creates and Fernet-encrypts the server's
master xprv before first launch.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/generate_master_key.py

@@ -0,0 +1,16 @@
+"""One-time ops bootstrap: generate and encrypt the server's master xprv.
+
+Usage:
+    python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
+    # put that value in XPRV_ENCRYPTION_KEY (.env), then:
+    python scripts/generate_master_key.py
+"""
+
+from app.config import settings
+from app.wallet.hd import generate_master_key
+
+if __name__ == "__main__":
+    if not settings.xprv_encryption_key:
+        raise SystemExit("XPRV_ENCRYPTION_KEY is not set")
+    generate_master_key()
+    print(f"Master key written (encrypted) to {settings.master_key_path}")