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/schemas/encounter.py

@@ -17,6 +17,7 @@ class EncounterUpdate(CamelModel):
     nickname: str | None = None
     status: str | None = None
     faint_level: int | None = None
+    death_cause: str | None = None
 
 
 class EncounterResponse(CamelModel):
@@ -28,6 +29,7 @@ class EncounterResponse(CamelModel):
     status: str
     catch_level: int | None
     faint_level: int | None
+    death_cause: str | None
     caught_at: datetime