Add game_id FK to BossBattle for version-exclusive bosses

Version-exclusive bosses (e.g., Bea in Sword, Allister in Shield) were
using the section field to indicate which game they belong to. This adds
a proper game_id foreign key so the API can filter bosses per game,
keeping section free for visual grouping like "Main Story".

- Alembic migration adds nullable game_id column with FK and index
- API list_bosses filters by game_id unless ?all=true is passed
- Seed data updated to use game_slug instead of section overloading
- Admin form adds "Game (version exclusive)" dropdown
- Export endpoints include game_slug for exclusive bosses

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.beans/nuzlocke-tracker-zmvy--add-game-id-field-to-bossbattle-for-version-exclus.md

@@ -0,0 +1,11 @@
+---
+# nuzlocke-tracker-zmvy
+title: Add game_id field to BossBattle for version-exclusive bosses
+status: completed
+type: feature
+priority: normal
+created_at: 2026-02-14T09:47:40Z
+updated_at: 2026-02-14T09:52:59Z
+---
+
+Add a proper game_id FK to BossBattle so version-exclusive bosses can be filtered per game instead of overloading the section field.