feat(etl): port handle_comment/event/share triggers (parity 1D)

[raymondjacobson]

Summary

Stack 1D of the trigger-port plan. Migration 0020 adds:

• New aggregate columns: `aggregate_track.{comment_count,share_count}`, `aggregate_playlist.share_count`, `aggregate_user.track_share_count` (apps#0145).
• `handle_comment` — maintains `aggregate_track.comment_count` from non-deleted, visible comments.
• `handle_event` — creates `fan_remix_contest_started` notifications for followers + favoriters when a remix_contest event is added on a public track.
• `handle_share` — maintains `aggregate_track.share_count` or `aggregate_playlist.share_count` and `aggregate_user.track_share_count`.

Skipped

`handle_supporter_rank_up` is intentionally not ported. It depends on `user_bank_txs` and `supporter_rank_ups` tables which are part of the Solana indexer (explicitly out of scope per the parity plan).

Stack context

Stacked on #240 (1C — content triggers).

• 1A — aggregate tables (feat(etl): add aggregate and milestone tables (parity 1A) #238)
• 1B — social triggers (feat(etl): port social triggers — handle_follow/save/repost (parity 1B) #239)
• 1C — content triggers (feat(etl): port handle_user/track/playlist triggers (parity 1C) #240)
• 1D (this PR) — secondary triggers
• handle_play deferred until apps-shaped `plays` table lands

Test plan

Three DB-backed tests in secondary_triggers_test.go:

• `TestTrigger_HandleComment_TicksCommentCount` — comment insert ticks aggregate_track.comment_count
• `TestTrigger_HandleShare_TicksTrackShareCount` — share insert ticks track + user share counts
• `TestTrigger_HandleEvent_RemixContestNotifiesFollowers` — remix_contest event creates fan_remix_contest_started notification for followers
• All 1B + 1C + 1D trigger tests pass together (no regressions)
• `go build ./...` clean

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