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.yml

@@ -2,7 +2,10 @@ name: crypto_trading
 
 services:
   backend:
-    build: ./backend
+    build:
+      context: ./backend
+      args:
+        BUILD_VERSION: ${CRYPTO_BUILD_VERSION:-unknown}
     image: crypto_trading-backend
     container_name: crypto-trading-backend
     restart: unless-stopped
@@ -56,7 +59,10 @@ services:
       - internal
 
   frontend:
-    build: ./frontend
+    build:
+      context: ./frontend
+      args:
+        BUILD_VERSION: ${CRYPTO_BUILD_VERSION:-unknown}
     image: crypto_trading-frontend
     container_name: crypto-trading-frontend
     restart: unless-stopped