Add ability to end a run as victory or defeat

Add End Run button to run dashboard with a confirmation modal offering
Victory/Defeat choice. Backend auto-sets completedAt timestamp and
validates only active runs can be ended. Ended runs show a completion
banner with date and duration, rename team to "Final Team", and hide
encounter logging and status change interactions.

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

backend/src/app/api/runs.py

@@ -1,3 +1,5 @@
+from datetime import datetime, timezone
+
 from fastapi import APIRouter, Depends, HTTPException, Response
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -73,6 +75,15 @@ async def update_run(
         raise HTTPException(status_code=404, detail="Run not found")
 
     update_data = data.model_dump(exclude_unset=True)
+
+    # Auto-set completed_at when ending a run
+    if "status" in update_data and update_data["status"] in ("completed", "failed"):
+        if run.status != "active":
+            raise HTTPException(
+                status_code=400, detail="Only active runs can be ended"
+            )
+        update_data["completed_at"] = datetime.now(timezone.utc)
+
     for field, value in update_data.items():
         setattr(run, field, value)