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>

backend/app/core/security.py

@@ -15,14 +15,19 @@ def verify_password(plain: str, hashed: str) -> bool:
 
 
 def create_access_token(subject: str) -> str:
-    expire = datetime.now(timezone.utc) + timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES)
+    now = datetime.now(timezone.utc)
+    expire = now + timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES)
     return jwt.encode(
-        {"sub": subject, "exp": expire},
-        settings.SECRET_KEY,
+        {"sub": subject, "exp": expire, "iat": now},
+        settings.JWT_PRIVATE_KEY,
         algorithm=settings.ALGORITHM,
     )
 
 
 def decode_access_token(token: str) -> str:
-    payload = jwt.decode(token, settings.SECRET_KEY, algorithms=[settings.ALGORITHM])
+    payload = jwt.decode(
+        token,
+        settings.JWT_PUBLIC_KEY,
+        algorithms=[settings.ALGORITHM],
+    )
     return payload["sub"]