Self-heal a stale cached bundle via a build version

Phones kept running old JS because their cached index.html pointed at an old
hashed asset. Both images now bake the commit SHA as BUILD_VERSION; the backend
exposes it at GET /api/version and the client reloads once with a cache-busting
query when its own baked version differs. Guarded by sessionStorage so it cannot
loop. /api/version is unauthenticated like /api/health.

backend/app/api/routes.py

@@ -63,6 +63,13 @@ def health() -> dict[str, str]:
     return {"status": "ok"}
 
 
+@router.get("/version")
+def version(request: Request) -> dict[str, str]:
+    """Build version of the running backend, used by the client to self-heal
+    from a stale cached bundle after a deploy."""
+    return {"version": getattr(request.app.state, "build_version", "unknown")}
+
+
 @router.get("/strategies", response_model=list[StrategyOut])
 def strategies() -> list[StrategyOut]:
     return [strategy_spec(cls) for cls in list_strategies()]