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/models/boss_battle.py

@@ -16,6 +16,7 @@ class BossBattle(Base):
     )
     name: Mapped[str] = mapped_column(String(100))
     boss_type: Mapped[str] = mapped_column(String(20))  # gym_leader, elite_four, champion, rival, evil_team, other
+    specialty_type: Mapped[str | None] = mapped_column(String(20), default=None)  # pokemon type specialty (e.g. rock, water)
     badge_name: Mapped[str | None] = mapped_column(String(100))
     badge_image_url: Mapped[str | None] = mapped_column(String(500))
     level_cap: Mapped[int] = mapped_column(SmallInteger)