feat: add multi-provider notification system with Telegram support

[GaIsBAX]

Summary

Add multi-provider notification system. Every time a webhook hits an endpoint, Webhix can fire a notification to configured channels. Currently supports Telegram; adding Discord or Slack later is one file + one line.

What's new

DB: hook_notification_channels table — one row per provider per endpoint, config stored as JSON.

API:

• GET /api/endpoints/:token/notifications — list configured channels
• PUT /api/endpoints/:token/notifications/:provider — save config
• DELETE /api/endpoints/:token/notifications/:provider — remove
• POST /api/endpoints/:token/notifications/:provider/test — send test message

CLI:

webhix notify list <token>
webhix notify telegram set <token> --bot-token "123:ABC..." --chat "1887325883"
webhix notify telegram test <token>
webhix notify telegram remove <token>

UI: Settings tab on endpoint page with Telegram fields (bot token, chat ID, optional proxy URL).

Extending

To add a new provider: create internal/notify/discord.go with a discordSend func, register it in internal/notify/provider.go, add internal/cli/notify/discord/ subpackage. Nothing else to touch.