Add pokemon status management with death tracking

Implement status change workflow (alive → dead) with confirmation modal,
death cause recording, and visual status indicators on pokemon cards.
Includes backend migration for death_cause field and graveyard view
on the run dashboard.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/app/models/encounter.py

@@ -17,6 +17,7 @@ class Encounter(Base):
     status: Mapped[str] = mapped_column(String(20))  # caught, fainted, missed
     catch_level: Mapped[int | None] = mapped_column(SmallInteger)
     faint_level: Mapped[int | None] = mapped_column(SmallInteger)
+    death_cause: Mapped[str | None] = mapped_column(String(100))
     caught_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now()
     )