feat(etl): add aggregate and milestone tables (parity 1A)

[raymondjacobson]

Summary

First step toward parity with discovery-provider's trigger-driven aggregation. Pure-DDL migration — no triggers, no handler changes.

Adds:

• `aggregate_user` (follower/following/track/repost/save/supporter counts)
• `aggregate_track` / `aggregate_playlist` (repost + save counts)
• `aggregate_plays` (lifetime listen count) + `aggregate_monthly_plays`
• `milestones` (follower/save threshold milestones)

Schema mirrors apps/packages/discovery-provider/ddl/migrations/0000_schema_failable.sql including the `idx_aggregate_user_follower_count` index added in apps#0126.

Stack context

This is PR 1/5 in the trigger-port stack. Stacked on #237 (stems/remixes).

• 1A (this PR) — aggregate + milestone tables
• 1B — social triggers (handle_follow, handle_save, handle_repost)
• 1C — content triggers (handle_track, handle_playlist, handle_play)
• 1D — wire notification creation into triggers
• 1E — secondary triggers (handle_comment, handle_event, handle_share, handle_supporter_rank_ups)

Test plan

• Migration up applies cleanly on fresh DB; `\dt aggregate_*; \dt milestones` shows all 6 tables.
• `\d aggregate_user` shows expected columns + PK + follower index.
• Migration down drops all tables cleanly with no errors.
• Existing entity_manager tests still pass.

🤖 Generated with Claude Code