Add error handling for timestamp parsing

[gita-agents]

Handle malformed database timestamps gracefully in _from_db_row to prevent ValueError exceptions.

Findings (1)

• [LOW] bug at backend/app/models/user.py:197 — Potential ValueError in _from_db_row when parsing timestamps: datetime.fromisoformat() on lines 197-198 and 199-200 will raise ValueError if the database returns a timestamp in an unexpected format, with no error handling.
  Fix sketch: Wrap datetime.fromisoformat() calls in try-except blocks to handle malformed timestamps gracefully, or validate timestamp format before parsing.

---

Generated by GITA v0.1.0 onboarding agent against amass. Milestone confidence: 0.80.