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/loader.py

@@ -220,6 +220,7 @@ async def upsert_bosses(
             version_group_id=version_group_id,
             name=boss["name"],
             boss_type=boss["boss_type"],
+            specialty_type=boss.get("specialty_type"),
             badge_name=boss.get("badge_name"),
             badge_image_url=boss.get("badge_image_url"),
             level_cap=boss["level_cap"],
@@ -232,6 +233,7 @@ async def upsert_bosses(
             set_={
                 "name": boss["name"],
                 "boss_type": boss["boss_type"],
+                "specialty_type": boss.get("specialty_type"),
                 "badge_name": boss.get("badge_name"),
                 "badge_image_url": boss.get("badge_image_url"),
                 "level_cap": boss["level_cap"],