feat(notifications): optional Telegram forum topic via message_thread_id (#1518)

Telegram groups with Topics enabled always received notifications in the
General topic, since only Bot Token and Chat ID were configurable. Splitting
notifications per printer meant running a separate chat for each one.

The Telegram provider now takes an optional Forum Topic ID - the last number
in a topic's link, t.me/c/1234567890/25 - and routes its messages there. Left
empty, nothing changes.

The value is coerced to an int once in _send_telegram and attached to both the
sendMessage JSON body and the sendPhoto form data. That ordering matters:
Telegram rejects a string message_thread_id in the JSON body while accepting
one in the multipart call, so passing the raw form value through would have
worked for thumbnail notifications and 400'd for plain-text ones. A
non-numeric value is rejected in the form and again server-side before any
request goes out.

No migration - provider config is a JSON blob.

Adds Forum Topic ID plus help text to the Telegram section of the provider
dialog, translated in all 13 locales. Backend tests cover omitted / blank /
int-typed / non-numeric values and both send paths; frontend tests cover the
field being optional, absent for other providers, round-tripping on save, and
blocking save on a bad value.

static/index.html

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+    <script type="module" crossorigin src="/assets/index-utEp4da9.js"></script>
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