feat(etl): port social triggers — handle_follow/save/repost (parity 1B)

[raymondjacobson]

Summary

Stack 1B of the trigger-port plan. Migration 0018 adds:

• `aggregate_user.score` column (used by trigger shadowban check; default 0 = not shadowbanned, matching apps' bootstrap state).
• `handle_follow` — maintains `aggregate_user.{follower_count,following_count}`, inserts milestone rows + follower-count and follow notifications.
• `handle_save` — maintains `aggregate_track.save_count` / `aggregate_playlist.save_count` / `aggregate_user.track_save_count`, plus milestone, save, save-of-repost, and remix-cosign notifications.
• `handle_repost` — maintains repost counts on `aggregate_track` / `aggregate_playlist` / `aggregate_user`, plus milestone, repost, repost-of-repost, and remix-cosign notifications.

Stubs

`handle_save` references two tables that don't exist in go-openaudio yet:

• `usdc_purchases` (Solana-side; intentionally out of scope per the parity plan)
• `playlist_tracks` (Stack 2A)

Both `is_purchased` and `is_containing_album_purchased` are stubbed to `false`. Without Solana indexing nothing is purchased, so this is the correct value for the current environment. When Stack 2A lands, the `playlist_tracks` JOIN can be re-introduced (it only matters once `usdc_purchases` exists, which is a separate Solana effort).

Stack context

Stacked on #238 (1A — aggregate tables).

• 1A — aggregate + milestone tables (feat(etl): add aggregate and milestone tables (parity 1A) #238)
• 1B (this PR) — social triggers (handle_follow/save/repost)
• 1C — content triggers (handle_track/playlist/play)
• 1D — wire notification creation into triggers
• 1E — secondary triggers (handle_comment/event/share/supporter_rank_ups)

Test plan

Six DB-backed tests in social_triggers_test.go:

• `TestTrigger_HandleFollow_TicksAggregateCounts` — follower/following counts increment
• `TestTrigger_HandleFollow_CreatesNotification` — `type='follow'` row appears in `notification`
• `TestTrigger_HandleSave_TicksTrackSaveCount` — track + user save counts increment
• `TestTrigger_HandleSave_CreatesNotification` — `type='save'` notification fires
• `TestTrigger_HandleRepost_TicksRepostCounts` — track + user repost counts increment
• `TestTrigger_HandleUnfollow_DecrementsCounts` — counts go back down
• Existing entity_manager test suite still passes (no regressions from added triggers)
• `go build ./...` clean

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