Refactor token handling and introduce token revocation logic

Updated `decode_token` for stricter validation of token claims and explicit error handling. Added utilities for token revocation and verification, improving

backend/app/auth/utlis.py

@@ -0,0 +1,16 @@
+# auth/utils.py
+from sqlalchemy.ext.asyncio import AsyncSession
+from app.models.token import RevokedToken
+
+
+async def revoke_token(jti: str, token_type: str, user_id: str, db: AsyncSession):
+    """Revoke a token by adding its `jti` to the database."""
+    revoked_token = RevokedToken(jti=jti, token_type=token_type, user_id=user_id)
+    db.add(revoked_token)
+    await db.commit()
+
+
+async def is_token_revoked(jti: str, db: AsyncSession):
+    """Check if a token with the given `jti` is revoked."""
+    result = await db.get(RevokedToken, jti)
+    return result is not None