Add conditional boss battle teams (variant teams by condition)

Wire up the existing condition_label column on boss_pokemon to support
variant teams throughout the UI. Boss battles can now have multiple team
configurations based on conditions (e.g., starter choice in Gen 1).

- Add condition_label to BossPokemonInput schema (frontend + backend bulk import)
- Rewrite BossTeamEditor with variant tabs (Default + named conditions)
- Add variant pill selector to BossDefeatModal team preview
- Add BossTeamPreview component to RunEncounters boss cards
- Fix MissingGreenlet error in set_boss_team via session.expunge_all()
- Fix PokemonSelector state bleed between tabs via composite React key
- Add Alembic migration for condition_label column

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

backend/src/app/schemas/boss.py

@@ -9,6 +9,7 @@ class BossPokemonResponse(CamelModel):
     pokemon_id: int
     level: int
     order: int
+    condition_label: str | None
     pokemon: PokemonResponse
 
 
@@ -73,6 +74,7 @@ class BossPokemonInput(CamelModel):
     pokemon_id: int
     level: int
     order: int
+    condition_label: str | None = None
 
 
 class BossResultCreate(CamelModel):

backend/src/app/schemas/pokemon.py

@@ -200,6 +200,7 @@ class BulkBossPokemonItem(BaseModel):
     pokeapi_id: int
     level: int
     order: int
+    condition_label: str | None = None
 
 
 class BulkBossItem(BaseModel):