feat: add Gotify self-hosted push notification provider (#6423)

[qiuluoli]

Implements issue #6423: Gotify self-hosted push notification provider for Keep.

What

• Gotify push notification provider
• Same pattern as other Keep notification providers

Features

• Self-hosted push notifications via Gotify API
• Severity-to-priority mapping (critical=10, high=8, warning=5, info=3, low=1)
• Simple POST /message?token= API
• Auth: server URL + application token

/attempt #6423

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[shahargl]

N/A