feat: make level field optional in boss defeat modal

Remove the level input from the boss defeat modal since the app doesn't
track levels elsewhere. Team selection is now just checkboxes without
requiring level entry.

- Remove level input UI from BossDefeatModal.tsx
- Add alembic migration to make boss_result_team.level nullable
- Update model and schemas to make level optional (defaults to null)
- Conditionally render level in boss result display

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

backend/src/app/models/boss_result_team.py

@@ -14,7 +14,7 @@ class BossResultTeam(Base):
     encounter_id: Mapped[int] = mapped_column(
         ForeignKey("encounters.id", ondelete="CASCADE"), index=True
     )
-    level: Mapped[int] = mapped_column(SmallInteger)
+    level: Mapped[int | None] = mapped_column(SmallInteger, nullable=True)
 
     boss_result: Mapped[BossResult] = relationship(back_populates="team")
     encounter: Mapped[Encounter] = relationship()