harden(deploy): add app/frontend healthchecks + readiness-gate Caddy

Caddy depended on app/frontend with no readiness gate, so it could proxy to a
not-yet-listening upstream and surface brief 502s on boot/restart. Add
wget-based healthchecks (busybox wget ships in both alpine runtime images):
app hits the unauthenticated /health, frontend hits / (SSR returns 200; the
redirect is client-side). app uses a 30s start_period to cover boot-time
migrations. Caddy now depends_on both with condition: service_healthy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -35,6 +35,13 @@ services:
       - media_data:/media
     expose:
       - "3000"
+    # /health is unauthenticated; start_period covers the boot-time migrations.
+    healthcheck:
+      test: ["CMD", "wget", "-q", "-O", "/dev/null", "http://localhost:3000/health"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 30s
 
   frontend:
     build:
@@ -43,9 +50,16 @@ services:
     restart: unless-stopped
     env_file: .env
     depends_on:
-      - app
+      app:
+        condition: service_healthy
     expose:
       - "3001"
+    healthcheck:
+      test: ["CMD", "wget", "-q", "-O", "/dev/null", "http://localhost:3001/"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 15s
 
   caddy:
     image: caddy:2-alpine
@@ -56,9 +70,13 @@ services:
     volumes:
       - ./Caddyfile:/etc/caddy/Caddyfile:ro
       - caddy_data:/data
+    # Gate on readiness so Caddy doesn't proxy to a not-yet-listening upstream
+    # (which otherwise shows brief 502s on boot/restart).
     depends_on:
-      - app
-      - frontend
+      app:
+        condition: service_healthy
+      frontend:
+        condition: service_healthy
 
 volumes:
   postgres_data: