Add optional specialty type field to boss battles

Gym leaders, Elite Four, and champions can now have a Pokemon type
specialty (e.g. Rock, Water). Shown as a type image badge on boss
cards in the run view, and editable via dropdown in the admin form.
Includes migration, export, and seed pipeline support.

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

backend/src/app/schemas/boss.py

@@ -17,6 +17,7 @@ class BossBattleResponse(CamelModel):
     version_group_id: int
     name: str
     boss_type: str
+    specialty_type: str | None
     badge_name: str | None
     badge_image_url: str | None
     level_cap: int
@@ -43,6 +44,7 @@ class BossResultResponse(CamelModel):
 class BossBattleCreate(CamelModel):
     name: str
     boss_type: str
+    specialty_type: str | None = None
     badge_name: str | None = None
     badge_image_url: str | None = None
     level_cap: int
@@ -56,6 +58,7 @@ class BossBattleCreate(CamelModel):
 class BossBattleUpdate(CamelModel):
     name: str | None = None
     boss_type: str | None = None
+    specialty_type: str | None = None
     badge_name: str | None = None
     badge_image_url: str | None = None
     level_cap: int | None = None