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/seeds/run.py

@@ -429,6 +429,7 @@ async def _export_bosses(session: AsyncSession, vg_data: dict):
             {
                 "name": b.name,
                 "boss_type": b.boss_type,
+                "specialty_type": b.specialty_type,
                 "badge_name": b.badge_name,
                 "badge_image_url": b.badge_image_url,
                 "level_cap": b.level_cap,