Add YouTube notifications to the notify hybrid group

[goodmorningkay]

Summary

Extend the notify hybrid group with YouTube channel follows. Users can now follow a YouTube channel and choose whether to be notified for uploads, live streams, or both, in a server or by DM — mirroring the existing Twitch/anime command structure.

What was added

Commands (all hybrid, guild + DM)

• notify add youtube <channel> [events] — events = uploads / live / both (default both), with channel-search autocomplete
• notify youtube <channel> [events] — shorthand for the above
• notify remove youtube <follow> — by follow ID, channel name, or channel URL
• notify mention youtube <follow> [mention] — set/clear a role or @everyone mention
• notify events youtube <follow> <events> — change an existing follow's event type
• notify list youtube — list followed channels; YouTube also appears as a section in notify list
• notify channel youtube — guild-only routing to a server text channel
• Autocomplete for channel search, followed channels, mention roles, and text channels

Backend (extensions/events/youtube.py)

• _dispatch_youtube_item now selects user_id, mention_role_id, mention_everyone
• New _youtube_owner + _youtube_destination resolve a follow's channel or DM destination
• Delivery dedup claims by guild or user scope via partial unique indexes
• Announcements honor per-follow mentions (notify_allowed_mentions) and can DM the owner
• New search_youtube_channels (Google API) for add autocomplete

Migration 0093_youtube_notify_follows.sql

• youtube_follows: adds user_id, an id identity primary key, mention_role_id/mention_everyone; the (guild_id, youtube_channel_id) primary key becomes a UNIQUE constraint so DM rows fit while the website's ON CONFLICT (guild_id, youtube_channel_id) inserts keep working
• youtube_announcement_deliveries: adds user_id and scope-keyed partial unique indexes for DM delivery dedup

Other

• youtube_follows added to USER_ID_TABLES in core/privacy.py for GDPR wipes
• notify list / landing text updated to mention YouTube

Testing

• New tests/test_notify_youtube.py covering event mapping, list rendering, owner/destination resolution, and guild vs DM delivery-claim scoping
• Full suite: 1328 tests pass
• Migration validated against Postgres 18 (guild + DM inserts, ON CONFLICT, owner check, id auto-increment, delivery dedup by both scopes, and the runtime claim SQL partial-index match)
• Cog-load smoke test confirms all youtube subcommands register under add/remove/mention/events/list/channel; notify add youtube exposes channel + events params
• Ruff clean

Notes

• notify list section order is Twitch → YouTube → Anime
• The short/community event types still work via the website API but are not exposed as command choices