feat(notifications): handle_comment_remix_contest_update trigger

[raymondjacobson]

Summary

Adds the missing fourth contest notification — remix_contest_update. Sibling of the three already in this repo:

Notification	File
fan_remix_contest_started	handle_event.sql
artist_remix_contest_submissions	handle_track.sql
fan_remix_contest_submission	handle_track.sql
remix_contest_update	handle_comment_remix_contest_update.sql (this PR)

Triggers when the contest host posts a TOP-LEVEL comment on their own remix_contest event. Fans out one notification per active event subscriber (excluding the host). Mirrors AudiusProject/apps#14159's python create_comment block.

Recipient set + data shape

Matches the python indexer 1:1 so existing frontend renderers (the web + mobile components shipped in apps#14159) consume it without changes.

• Recipient set: subscriptions WHERE entity_type='Event' AND user_id=event_id AND is_current AND NOT is_delete, excluding the host.
• group_id: remix_contest_update:{comment_id}:event:{event_id}
• specifier: {recipient_id} (per-recipient row; (group_id, specifier) is the unique constraint)
• data: {event_id, entity_id (= contest's parent track), entity_user_id (= host), comment_id}

Why DEFERRABLE INITIALLY DEFERRED

The "is_reply" check looks at comment_threads. The indexer inserts the comment_threads row AFTER the comments row in the same transaction. A plain AFTER INSERT trigger on comments fires before comment_threads is visible and would misclassify every reply as top-level.

Using a CONSTRAINT TRIGGER ... DEFERRABLE INITIALLY DEFERRED fires the trigger at commit time, by which point both rows are visible. The trigger function checks EXISTS(SELECT 1 FROM comment_threads WHERE comment_id = NEW.comment_id) to detect replies.

Standard exception when others wrapper so a bug in the notification path never aborts the user's comment insert.

Tests

Three tests in api/v1_event_comments_notification_test.go, all exercising the trigger via app.writePool:

• TestRemixContestUpdate_NotifiesEventSubscribers — host top-level comment on own event: one notification per non-host subscriber, with the expected group_id/specifier/data/user_ids shape. Host's own (self-)subscription is excluded.
• TestRemixContestUpdate_SkipsReplies — host posts a reply (both comments and comment_threads INSERTs in the same transaction): zero notifications. Exercises the deferred-trigger contract.
• TestRemixContestUpdate_SkipsNonHostComments — random non-host commenter on the event: zero notifications.

Companion change

OpenAudio/go-openaudio#311 ports the auto_subscribe_to_contest_on_submission half of apps#14159 to the Go ETL — without those subscriptions rows landing, the recipient set this trigger fans out to (and the two in handle_track.sql) would be empty.

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