feat: add Discord webhook notification channel

[SirKitboard]

Discord is a common hub for home server enthusiasts, and currently there's no way to get outfit notifications there without routing through a separate service. This adds Discord as a first-class notification channel alongside the existing ntfy, Mattermost, and email options.

Changes

Backend

• DiscordConfig schema with webhook URL validation
• DiscordProvider that sends rich embeds (title, description, highlights, tip, footer link)
• Wired into NotificationDispatcher and test_setting alongside existing channels

Frontend

• Discord option in the Add Channel dialog with a webhook URL field and setup hint
• Channel card display consistent with other channels

Refactor (touched while adding Discord)

• Extracted _get_highlights() and _weather_text() as shared helpers on NotificationDispatcher — these were duplicated across all four notification builders. Also fixed an inconsistency where Mattermost was showing C instead of °C.

Setup

In Discord: channel settings → Integrations → Webhooks → New Webhook → copy URL.