Root cause

{{ restock.price }} (and the other restock.* tokens) are only added to the
notification context for restock watches, in
processors/restock_diff extra_notification_token_values(). The base token set in
NotificationContextData.__init__ (changedetectionio/notification_service.py) has no
restock key, so for a non-restock watch, or for a system-wide notification body, the
token is undefined. That produces two failures:

1. Send time: rendering a body containing {{ restock.price }} for a non-restock watch
   raises jinja2 UndefinedError: 'restock' is undefined (the exact error in 'restock' is undefined #3490).
2. Save time: ValidateJinja2Template builds its valid-token set from the same defaults,
   so a system-wide body using {{ restock.price }} is rejected unless a restock watch
   happens to exist.

Invariant

A notification token that is valid in a per-watch body should also be a valid,
non-crashing token in a system-wide body, independent of the current watch inventory.

Fix

Add a safe-empty 'restock': {} to the NotificationContextData default token set.
Restock watches still override it with the real price object via
extra_notification_token_values(), so their notifications are unchanged. Every other
watch now renders {{ restock.price }} as empty rather than raising, and system-wide
bodies validate.

This is intentionally the narrow token-availability fix for #3490, not the separate
restock-data storage change noted in the issue thread.

How I verified

In a clean python:3.11-slim Docker container against master HEAD (ee84ece):

• Reproduced both failures on master: render('{{ restock.price }}', **NotificationContextData())
  raises UndefinedError, and ValidateJinja2Template raises ValidationError, both
  'restock' is undefined.
• Added tests/test_notification_restock_token.py covering the default token, the
  send-time render, and the save-time validation. It fails on master (3 failed) and
  passes with the fix (3 passed).
• Ran the existing tests/llm/test_notification_tokens.py and
  tests/llm/test_llm_restock_plugin.py with the fix: 36 passed.

I did not run the full browser/selenium/playwright suite or the SMTP integration tests;
this change only touches the in-memory default token dict.

Run: pytest tests/test_notification_restock_token.py

AI assistance was used to help write this change; I reproduced and verified it as
described above.

Fixes #3490