feat: add LINE Notify provider for KeepHQ (#6426)

[wsimon1982]

Description

Fügt einen neuen LINE Notify Provider für KeepHQ hinzu (Issue #6426)

Changes

• LineNotifyProvider: Neuer Provider-Klasse die von BaseProvider erbt
• Auth: Bearer-Token Authentication über Authorization: Bearer <token> Header
• _notify: Sendet Messages an LINE Notify API (https://notify-api.line.me/api/notify)
• test(): Verbindungstest mit Dummy-Message
• validate_config(): Prüft ob access_token konfiguriert ist

API Details

• Endpoint: https://notify-api.line.me/api/notify
• Auth: Bearer <access_token>
• Token erstellen: https://notify-bot.line.me/my/

Note

LINE Notify wurde am 31. März 2025 eingestellt, die API funktioniert aber weiterhin.
Für neue Projekte wird die LINE Messaging API (https://developers.line.biz/) empfohlen.

Test

In Keep konfigurieren:

1. Settings → Providers → Add Provider → LINE Notify
2. access_token eintragen
3. Test-Button für Verbindungstest

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[wsimon1982]

Fixes #6426
Closes #6426