fix(playlists): allow the 5 discovery-mix variants in DB constraints

HOTFIX for v2026.05.15.0. R3 added deep_cuts/rediscover/new_for_you/
on_this_day/first_listens producers + registry entries but not their
system_variant values to the playlists_kind_variant_consistent /
playlists_seed_consistent CHECK constraints (0021's whitelist).
insertSystemPlaylist for any new mix → SQLSTATE 23514, and since
BuildSystemPlaylists is one all-or-nothing txn that aborts the
ENTIRE build — For-You/Discover refresh 500s and the daily lazy
build fails too. System playlists are fully broken in prod.

Migration 0028 drops + re-adds both constraints with all five new
seedless variants (mirrors the 0021 drop-and-readd pattern).
CHECK-only — sqlc/dbq unaffected (regen produced no drift).

Bumps to 2026.5.15+7 for the v2026.05.15.1 patch release.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/db/migrations/0028_discovery_mix_variants.down.sql

@@ -0,0 +1,24 @@
+-- Reverts to the 0021 constraint set (for_you, songs_like_artist,
+-- discover only). Fails if any playlists row still has one of the
+-- five new system_variant values — delete/rebuild system playlists
+-- first if rolling back.
+
+ALTER TABLE playlists DROP CONSTRAINT IF EXISTS playlists_kind_variant_consistent;
+ALTER TABLE playlists
+    ADD CONSTRAINT playlists_kind_variant_consistent CHECK (
+        (kind = 'user'   AND system_variant IS NULL)
+        OR
+        (kind = 'system' AND system_variant IN ('for_you', 'songs_like_artist', 'discover'))
+    );
+
+ALTER TABLE playlists DROP CONSTRAINT IF EXISTS playlists_seed_consistent;
+ALTER TABLE playlists
+    ADD CONSTRAINT playlists_seed_consistent CHECK (
+        (system_variant IS NULL                 AND seed_artist_id IS NULL)
+        OR
+        (system_variant = 'for_you'             AND seed_artist_id IS NULL)
+        OR
+        (system_variant = 'discover'            AND seed_artist_id IS NULL)
+        OR
+        (system_variant = 'songs_like_artist'   AND seed_artist_id IS NOT NULL)
+    );