Bug: Empty Exception Handlers Swallow Errors

## Summary
Multiple `except Exception: pass` blocks swallow errors silently, making debugging impossible.

## Affected Locations
- `send_greeting()` - webhook operations
- `login_callback()` - OAuth errors  
- `join()` - guild join errors

## Fix
Always log the exception:
```python
except Exception:
    logger.exception("Operation failed")
```

Applies to: v2-fastapi-rewrite PR
