fix: make Notifier accept configured service instances

[pcbeingused333]

Problem

Notifier.add_service / add_services / remove_service are annotated as taking type[Service] and call service():

def add_service(self, service: type[Service]) -> None:
    self._services.append(service())

Every built-in service requires constructor arguments (Discord(webhook_url), Resend(api_key, from_email, to_emails_raw), ...), so:

• notifier.add_services([Discord, Telegram]) — the form in the docstring — raises TypeError: __init__() missing 1 required positional argument.
• notifier.add_service(Discord(url)) raises TypeError: 'Discord' object is not callable.

remove_service is worse: it builds a new instance and removes it by ==, so it never matches an added service (no __eq__ on the ABC) and always raises "not found".

Notifier has no tests and no internal callers, so nothing pinned the broken behavior.

Fix

add_service / add_services / remove_service take configured instances. add_service raises a clear TypeError if handed a class by mistake; remove_service removes the instance directly.

Tests

New tests/unit/notification/test_notifier.py — 8 cases: add/notify, add_services with instances, rejecting a class, empty-notify no-op, a failing service not blocking the others, remove, remove-missing, clear. 7 of the 8 fail on main. uv run pytest tests/unit/notification is green (24 passed); ruff check / ruff format clean.

[pcbeingused333]

The pre-commit check is red on the ty hook, but that's pre-existing on main (commit 0d1862e fails the same job, on fenn/experimental/vision/normalize.py and fenn/logging.py — neither touched here). ruff check / ruff format pass; the four test jobs are the relevant ones.