Add healthcheck to Redis (redis-cli ping with auth)

Tier-1 health visibility for the shared Redis. Uses redis-cli with
the password from the mounted secret, fails on anything but PONG.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

infra/redis/docker-compose.yml

@@ -18,6 +18,13 @@ services:
     security_opt:
       - no-new-privileges:true
 
+    healthcheck:
+      test: ["CMD-SHELL", "redis-cli -a \"$$(cat /run/secrets/redis_password)\" --no-auth-warning ping | grep -q PONG"]
+      interval: 30s
+      timeout: 5s
+      retries: 5
+      start_period: 15s
+
 networks:
   backend_net:
     external: true