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.

docker-compose.local.yml

@@ -5,7 +5,10 @@ name: crypto_trading_local
 
 services:
   backend:
-    build: ./backend
+    build:
+      context: ./backend
+      args:
+        BUILD_VERSION: ${BUILD_VERSION:-unknown}
     container_name: crypto-trading-backend-local
     restart: unless-stopped
     environment:
@@ -37,7 +40,10 @@ services:
       - "8000:8000"
 
   frontend:
-    build: ./frontend
+    build:
+      context: ./frontend
+      args:
+        BUILD_VERSION: ${BUILD_VERSION:-unknown}
     container_name: crypto-trader-frontend-local
     restart: unless-stopped
     depends_on: