Switch JWT signing from HS256 to RS256 (4096-bit RSA)

- Replace symmetric SECRET_KEY with JWT_PRIVATE_KEY / JWT_PUBLIC_KEY (PEM)
- Add iat claim to every token
- Add expand_newlines validator in config for single-line .env PEM values
- Add scripts/generate_jwt_keys.py key-generation helper
- Update security-auditor agent JWT checklist with RS256 enforcement rules
- Mark RS256 as done in TODO.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/generate_jwt_keys.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+"""
+Generate a 4096-bit RSA key pair for JWT RS256 signing.
+
+Usage (from repo root):
+    python scripts/generate_jwt_keys.py
+
+Output: prints the two env var assignments to paste into backend/.env
+"""
+
+from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives.asymmetric import rsa
+from cryptography.hazmat.backends import default_backend
+
+
+def main() -> None:
+    private_key = rsa.generate_private_key(
+        public_exponent=65537,
+        key_size=4096,
+        backend=default_backend(),
+    )
+
+    private_pem = private_key.private_bytes(
+        encoding=serialization.Encoding.PEM,
+        format=serialization.PrivateFormat.TraditionalOpenSSL,
+        encryption_algorithm=serialization.NoEncryption(),
+    ).decode()
+
+    public_pem = private_key.public_key().public_bytes(
+        encoding=serialization.Encoding.PEM,
+        format=serialization.PublicFormat.SubjectPublicKeyInfo,
+    ).decode()
+
+    # Collapse newlines to \n so the value fits on one .env line
+    private_oneline = private_pem.replace("\n", "\\n")
+    public_oneline = public_pem.replace("\n", "\\n")
+
+    print("# Paste these lines into backend/.env\n")
+    print(f'JWT_PRIVATE_KEY="{private_oneline}"')
+    print(f'JWT_PUBLIC_KEY="{public_oneline}"')
+    print("\n# Keep JWT_PRIVATE_KEY secret — never commit it.")
+
+
+if __name__ == "__main__":
+    main()