fix(api): replace UUID type with str for Better-Auth nanoid user IDs (#98)

Better-Auth uses nanoid strings for user IDs, not UUIDs. Changed all
user_id parameter/return types in the API layer from UUID to str,
removed the obsolete UUID import where unused, and updated the
_validate_session_token return type accordingly.

Co-authored-by: CartSnitch Engineer Bot <cartnoreply@cartsnitch.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/auth/jwt.py

@@ -2,22 +2,21 @@
 
 from datetime import UTC, datetime, timedelta
 from typing import Any, cast
-from uuid import UUID
 
 from jose import JWTError, jwt
 
 from cartsnitch_api.config import settings
 
 
-def create_access_token(user_id: UUID) -> str:
+def create_access_token(user_id: str) -> str:
     expire = datetime.now(UTC) + timedelta(minutes=settings.jwt_access_token_expire_minutes)
-    payload = {"sub": str(user_id), "exp": expire, "type": "access"}
+    payload = {"sub": user_id, "exp": expire, "type": "access"}
     return cast(str, jwt.encode(payload, settings.jwt_secret_key, algorithm=settings.jwt_algorithm))
 
 
-def create_refresh_token(user_id: UUID) -> str:
+def create_refresh_token(user_id: str) -> str:
     expire = datetime.now(UTC) + timedelta(days=settings.jwt_refresh_token_expire_days)
-    payload = {"sub": str(user_id), "exp": expire, "type": "refresh"}
+    payload = {"sub": user_id, "exp": expire, "type": "refresh"}
     return cast(str, jwt.encode(payload, settings.jwt_secret_key, algorithm=settings.jwt_algorithm))