triggers: validate duplicate cron IDs and webhook paths in TriggerManager.update()

[nathanschram]

Source: CodeRabbit feedback on #309 (deferred from the v0.35.1 release fix PR #311)

Current behaviour: `TriggerManager.update()` builds the webhook lookup as:

```python
self._webhooks_by_path = {wh.path: wh for wh in settings.webhooks}
```

If two webhooks share the same `path`, the later one silently overwrites the earlier one — the user has no warning that one of their webhooks is unreachable.

`remove_cron()` similarly only removes the first match and returns; if duplicate IDs exist the second one stays in place.

Suggested fix:

• In `TriggerManager.update()`, raise `ValueError` if any webhook `path` or any cron `id` is duplicated (with both occurrence locations in the message)
• Validate at config parse time too if possible (TriggerSettings construction)
• Add tests for the duplicate-detection paths

Why deferred: Not a functional regression for correctly-configured users, just defensive hardening that masks user typos. Release blocker fixes were prioritised.

Files:

• `src/untether/triggers/manager.py` (lines 47, 128-144)
• `src/untether/triggers/settings.py` (validation hook)
• `tests/test_trigger_manager.py` (new tests)