feat(notifications): comment, mention, thread, reaction, fan_club_text_post triggers

[raymondjacobson]

Summary

Adds the five comment-related notification triggers that apps' src/tasks/entity_manager/entities/comment.py creates directly during ManageEntity processing. The ETL handlers in go-openaudio/pkg/etl/processors/entity_manager/comment_*.go already write the source rows (comments, comment_mentions, comment_threads, comment_reactions) — but the user-facing notification rows had no Go equivalent. handle_comment.sql here previously only updated aggregate_track.comment_count.

Closes the largest remaining notification gap on the path to shutting off the Python discovery indexer (per the broader parity audit).

Triggers (one notification type per file)

File	Notification type	Source table	Pattern
handle_comment_notification.sql	comment	comments INSERT	Deferred (needs comment_threads + comment_mentions)
handle_comment_mention.sql	comment_mention	comment_mentions INSERT	Plain AFTER
handle_comment_thread.sql	comment_thread	comment_threads INSERT	Plain AFTER
handle_comment_reaction.sql	comment_reaction	comment_reactions INSERT	Plain AFTER
handle_fan_club_text_post.sql	fan_club_text_post	comments INSERT (FanClub)	Deferred (needs comment_threads)

Each follows the established repo pattern (one file per trigger, sibling of handle_comment_remix_contest_update.sql). All use ON CONFLICT DO NOTHING against notification.uq_notification(group_id, specifier) for dedup. Notification shape — specifier, group_id, data payload — matches apps verbatim so existing readers/clients keep working.

Why DEFERRABLE INITIALLY DEFERRED

`handle_comment_notification` and `handle_fan_club_text_post` need to look at `comment_threads` (to detect replies) and `comment_mentions` (to detect owner mentions). Those sibling rows are inserted after the `comments` row in the same indexer transaction. Same problem and same fix as `handle_comment_remix_contest_update.sql`.

Intentional gap: karma-based mute

Apps drops the notification when SUM(follower_count) of users who muted the commenter exceeds a configured threshold (1.7M prod, 4k dev). Not ported here — keeps triggers localized and the threshold lives in apps' config, not the DB. Worst-case impact is "spammy commenters notify a few more users than they would have under apps." If that becomes a real noise problem we fold it in.

Skip semantics (mirror apps)

• comment — skips self-comment, replies (they get comment_thread), owner-mentioned (they get comment_mention), comment_notification_settings.is_muted, muted_users
• comment_mention — skips self-mention, mention-muted-commenter, owner-mentioned-with-owner-mute
• comment_thread — skips self-reply, parent author muted thread or replier
• comment_reaction — skips self-react, comment-author mute, plus apps' track_owner_mention_mute when commenter is entity owner
• fan_club_text_post — only artist's own top-level posts fan out; recipients = followers ∪ artist-coin holders, excluding artist

Schema dump

Regeneration follows in a separate commit, matching 4da78ab for handle_comment_remix_contest_update.

Test plan

9 DB-backed tests against the test_api template, all passing locally:

• `TestCommentNotification_NotifiesTrackOwner` — happy path; correct group_id + payload
• `TestCommentNotification_SkipsSelfComment` — self-comment no-op
• `TestCommentNotification_SkipsReply` — comment + comment_threads in the same tx → deferred trigger correctly skips
• `TestCommentMention_NotifiesMentionedUser` — mentioned user gets `comment_mention` with full payload
• `TestCommentMention_SkipsWhenMentionedMutedCommenter` — `muted_users` gate
• `TestCommentThread_NotifiesParentAuthor` — parent author gets `comment_thread`, specifier = reply comment_id
• `TestCommentReaction_NotifiesCommentAuthor` — author gets `comment_reaction`, specifier = reacter user_id
• `TestFanClubTextPost_FansOutToFollowersAndCoinHolders` — followers ∪ coin holders, deduped via UNION; artist excluded
• `TestFanClubTextPost_SkipsFanComments` — only artist posts fan out

`go build ./...` and `go vet ./api/...` clean.

🤖 Generated with Claude Code