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/config.py

@@ -1,3 +1,4 @@
+from pydantic import field_validator
 from pydantic_settings import BaseSettings
 
 
@@ -6,12 +7,20 @@ class Settings(BaseSettings):
 
     DATABASE_URL: str = "postgresql+asyncpg://postgres:password@localhost:5432/destroying_sap"
 
-    SECRET_KEY: str = "change-me-in-production"
-    ALGORITHM: str = "HS256"
+    # RS256 asymmetric signing — generate keys with scripts/generate_jwt_keys.py
+    ALGORITHM: str = "RS256"
+    JWT_PRIVATE_KEY: str = ""  # PEM, required; set via env var
+    JWT_PUBLIC_KEY: str = ""   # PEM, required; set via env var
     ACCESS_TOKEN_EXPIRE_MINUTES: int = 60 * 8  # 8 hours — no permanent sessions
 
     CORS_ORIGINS: list[str] = ["http://localhost:5173"]
 
+    @field_validator("JWT_PRIVATE_KEY", "JWT_PUBLIC_KEY", mode="before")
+    @classmethod
+    def expand_newlines(cls, v: str) -> str:
+        """Allow PEM keys stored on a single line with literal \\n in .env."""
+        return v.replace("\\n", "\n") if isinstance(v, str) else v
+
     class Config:
         env_file = ".env"