feat(metrics): provenance as standard — pick_kind for all system mixes

The #1249 mechanism (stamp WHY a track is in the snapshot at build
time, freeze it onto the play at ingestion, break it down in metrics)
generalizes from a For You one-off to the standard for every system
mix (#1270):

- Migration 0039 widens both pick_kind CHECKs (drop + re-add in the
  same change) to taste/fresh + Discover's dormant/cross_user/random
  + tier1-3 for the rule-#131 eligibility ladders.
- GetForYouPickKindForTrack becomes GetSystemPickKindForTrack
  (user, variant, track); ingestion stamps any systemPlaylistSources
  play from its own variant's live snapshot, live + offline paths.
- Discover stamps its candidate bucket on discoverTrack before the
  interleave, making the 40/30/30 allocation measurable; dedup keeps
  the taking bucket's stamp.
- Metrics replace the for_you special-case with one pick-kind
  vocabulary — any family with attributed plays gets a breakdown,
  future stamping mixes need no metrics change.
- Web: breakdown sub-rows are now toggled per surface (collapsed by
  default) so eight stamping mixes don't swamp the card.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/db/migrations/0039_pick_kind_vocabulary.down.sql

@@ -0,0 +1,19 @@
+-- Rows stamped with the widened vocabulary must be cleared before the
+-- narrow CHECK can be re-added; NULL reads as "unattributed", which is
+-- the honest downgrade.
+UPDATE play_events SET pick_kind = NULL
+    WHERE pick_kind NOT IN ('taste', 'fresh');
+UPDATE playlist_tracks SET pick_kind = NULL
+    WHERE pick_kind NOT IN ('taste', 'fresh');
+
+ALTER TABLE play_events
+    DROP CONSTRAINT play_events_pick_kind_check;
+ALTER TABLE play_events
+    ADD CONSTRAINT play_events_pick_kind_check
+        CHECK (pick_kind IN ('taste', 'fresh'));
+
+ALTER TABLE playlist_tracks
+    DROP CONSTRAINT playlist_tracks_pick_kind_check;
+ALTER TABLE playlist_tracks
+    ADD CONSTRAINT playlist_tracks_pick_kind_check
+        CHECK (pick_kind IN ('taste', 'fresh'));

internal/db/migrations/0039_pick_kind_vocabulary.up.sql

@@ -0,0 +1,41 @@
+-- Provenance as standard (milestone #127, Scribe #1270).
+--
+-- #1249 introduced pick_kind as a For You one-off ('taste'/'fresh').
+-- The mechanism — stamp WHY a track is in the snapshot at build time,
+-- freeze it onto the play at ingestion, break it down in metrics — is
+-- now the standard for every system mix, so the CHECK vocabulary
+-- widens to cover:
+--
+--   taste / fresh          For You head vs freshness-injection tail
+--   dormant / cross_user / random
+--                          Discover's three candidate buckets — makes
+--                          the 40/30/30 allocation measurable instead
+--                          of a guess
+--   tier1 / tier2 / tier3  the tiered-eligibility ladder (project rule
+--                          #131): tier 1 pins the mix's exact desire,
+--                          each higher tier steps back a little. Tier
+--                          provenance is how we measure what the
+--                          step-back trades away.
+--
+-- Postgres CHECK whitelists can't be altered in place: DROP + re-ADD
+-- with the expanded list, in the same migration.
+
+ALTER TABLE playlist_tracks
+    DROP CONSTRAINT playlist_tracks_pick_kind_check;
+ALTER TABLE playlist_tracks
+    ADD CONSTRAINT playlist_tracks_pick_kind_check
+        CHECK (pick_kind IN (
+            'taste', 'fresh',
+            'dormant', 'cross_user', 'random',
+            'tier1', 'tier2', 'tier3'
+        ));
+
+ALTER TABLE play_events
+    DROP CONSTRAINT play_events_pick_kind_check;
+ALTER TABLE play_events
+    ADD CONSTRAINT play_events_pick_kind_check
+        CHECK (pick_kind IN (
+            'taste', 'fresh',
+            'dormant', 'cross_user', 'random',
+            'tier1', 'tier2', 'tier3'
+        ));