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/Dockerfile

@@ -5,6 +5,10 @@ ENV PYTHONUNBUFFERED=1 \
 
 WORKDIR /app
 
+# Build stamp exposed via /api/version so the frontend can detect a stale cache.
+ARG BUILD_VERSION=unknown
+ENV BUILD_VERSION=${BUILD_VERSION}
+
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt