feat(providers): add Gotify and Matrix notification providers

[ayoubil]

Summary

Adds Gotify (self-hosted push notifications) and Matrix (decentralized messaging) notification providers.

Closes #6423
Closes #6424

What is included

• keep/providers/gotify_provider/gotify_provider.py — Gotify push notification provider
• keep/providers/matrix_provider/matrix_provider.py — Matrix room notification provider
• Documentation and mint.json entries

Provider details

Gotify: Self-hosted push notification server. Auth: App Token + Server URL.
Matrix: Open decentralized messaging. Auth: Access Token + Homeserver URL + Room ID.

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