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/main.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import os
 from collections.abc import AsyncIterator
 from contextlib import asynccontextmanager
 
@@ -27,6 +28,9 @@ def create_app(
         await runtime.shutdown()
 
     app = FastAPI(title=settings.app_name, lifespan=lifespan)
+    # Baked at build time (see Dockerfile); the frontend compares its own
+    # version against this to self-heal from a stale cache.
+    app.state.build_version = os.environ.get("BUILD_VERSION", "unknown")
     app.add_middleware(
         CORSMiddleware,
         allow_origins=settings.cors_origin_list,