Add stats screen with backend endpoint and frontend page

Implements a dedicated /stats page showing cross-run aggregate statistics:
run overview with win rate, runs by game bar chart, encounter breakdowns,
top caught/encountered pokemon rankings, mortality analysis with death
causes, and type distribution. Backend endpoint uses aggregate SQL queries
to avoid N+1 fetching.

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

backend/src/app/api/routes.py

@@ -1,6 +1,6 @@
 from fastapi import APIRouter
 
-from app.api import encounters, evolutions, games, health, pokemon, runs
+from app.api import encounters, evolutions, games, health, pokemon, runs, stats
 
 api_router = APIRouter()
 api_router.include_router(health.router)
@@ -9,3 +9,4 @@ api_router.include_router(pokemon.router, tags=["pokemon"])
 api_router.include_router(evolutions.router, tags=["evolutions"])
 api_router.include_router(runs.router, prefix="/runs", tags=["runs"])
 api_router.include_router(encounters.router, tags=["encounters"])
+api_router.include_router(stats.router, prefix="/stats", tags=["stats"])