[P1] `limit` query param not validated — unhandled ValueError → 500

## Summary
In `api_post_history`:

```python
limit = min(int(request.args.get('limit', 20)), 100)
```

Passing `?limit=abc` throws an unhandled `ValueError`, resulting in a 500 error.

## Fix

```python
try:
    limit = min(int(request.args.get('limit', 20)), 100)
except (ValueError, TypeError):
    limit = 20
```

**Effort:** ~15 minutes

> Part of full audit: #1