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>

flutter_client/pubspec.yaml

@@ -1,7 +1,7 @@
 name: minstrel
 description: Minstrel mobile client
 publish_to: 'none'
-version: 2026.5.15+6
+version: 2026.5.15+7
 
 environment:
   sdk: '>=3.5.0 <4.0.0'