feat(etl): index stems, remixes, route_id, and migration playlist routes

[raymondjacobson]

Summary

Adds discovery-provider parity for the four entity-manager side-effects that were declared as constants in handler.go but not implemented (Stem, Remix, TrackRoute, PlaylistRoute). These aren't separate transaction types in discovery — they're side effects of Track/Playlist CRUD, and that's how they're wired here.

• stems table — populated on track create from metadata.stem_of.parent_track_id; cleaned up on track delete (rows where this track is the child).
• remixes table — refreshed on track create and update from metadata.remix_of.tracks[].parent_track_id (delete-all-then-insert).
• tracks.route_id — set on create and rebuilt on title change using helpers.create_track_route_id semantics (<handle>/<sanitized-title>). Falls back to a synthetic user-<id> handle for guest uploaders, matching apps#13818.
• playlist_routes migration row — on create, also insert the legacy <title>-<id> slug as is_current=false so old ID-suffixed URLs keep resolving (mirrors discovery's update_playlist_routes_table is_create=True).

Migration 0016_stems_remixes adds the stems and remixes tables (matching production's discovery-provider schema, including remixes_child_idx). New shared helpers live in track_relations.go.

Test plan

• Pure unit tests for slug/route_id/parent-id helpers (no DB)
• DB-backed integration tests covering each side effect:
  • TestTrackCreate_PopulatesStemsTable
  • TestTrackCreate_PopulatesRemixesTable
  • TestTrackCreate_SetsRouteID
  • TestTrackCreate_GuestUserSyntheticHandle
  • TestTrackUpdate_RefreshesRemixes
  • TestTrackUpdate_RebuildsRouteIDOnTitleChange
  • TestTrackDelete_RemovesStemsForChild
  • TestPlaylistCreate_AddsMigrationRoute
• go build ./... and go vet ./... clean
• Verify against production parity tool (pkg/etl/parity) once an indexed range with stems/remixes is available

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