feat(etl): port handle_user/track/playlist triggers (parity 1C)

[raymondjacobson]

Summary

Stack 1C of the trigger-port plan. Migration 0019 adds:

• `aggregate_user.total_track_count` column (apps#0143).
• `handle_user` — creates `aggregate_user` row on user insert. Other triggers (handle_playlist, handle_track, handle_follow) depend on this row existing.
• `handle_track` — maintains `aggregate_track` + `aggregate_user.{track_count, total_track_count}`, plus subscriber-create / remix / remix-contest notifications. Verbatim port; fires on INSERT OR UPDATE.
• `handle_playlist` — maintains `aggregate_playlist` + `aggregate_user.{playlist_count, album_count}`, plus subscriber-create and track-added-to-playlist notifications.

Deferred

`handle_play` — apps' trigger fires on a `plays` table matching apps' schema, but go-openaudio currently has only `etl_plays` (different schema). A follow-up PR will add the apps-shaped `plays` table and wire the play processor before the trigger lands.

Stripped (no reorg support per scoping)

`handle_playlist`'s `revert_blocks` lookup was removed (go-openaudio doesn't track reverts). With `old_row` treated as null:

• INSERT of public playlist (`is_private=false`) → delta=1 ✓
• INSERT of private playlist → delta=0 ✓

The private→public publish path in apps fires the trigger via INSERT of a new `is_current` row; go-openaudio's `playlist_update.go` does in-place UPDATE so this trigger won't fire on publish today. That's tracked in Stack 4A (`publish_scheduled_releases`).

Stack context

Stacked on #239 (1B — social 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 (this PR) — content triggers (handle_user / handle_track / handle_playlist)
• 1D — secondary triggers (handle_comment / handle_event / handle_share / handle_supporter_rank_ups)

Test plan

Six DB-backed tests in content_triggers_test.go:

• `TestTrigger_HandleTrack_InitializesAggregates` — aggregate_track row + user track counts increment
• `TestTrigger_HandleTrack_UnlistedDoesNotIncrementTrackCount` — unlisted track only ticks total_track_count
• `TestTrigger_HandleTrack_RemixCreatesNotification` — remix-of triggers a parent-owner notification
• `TestTrigger_HandlePlaylist_InitializesAggregates` — aggregate_playlist row + user playlist_count increments
• `TestTrigger_HandlePlaylist_AlbumIncrementsAlbumCount` — `is_album=true` ticks album_count instead
• `TestTrigger_HandlePlaylist_PrivateDoesNotIncrement` — `is_private=true` suppresses delta
• All 6 trigger tests + 1B's 6 tests pass together (no regressions)

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