Migrate from Fly.io to docker-server (img.pq.io)

- Add infra/docker-compose.yml, deploy-stack.sh, .env.example
- Remove Fly.io GitHub Actions workflow
- Build via EC2, deploy via Portainer + Caddy on Hetzner

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/docker-compose.yml

@@ -0,0 +1,19 @@
+services:
+  app:
+    image: registry.docker.pq.io/postconvert:latest
+    restart: unless-stopped
+    networks:
+      - postconvert-net
+    environment:
+      CONVERTER_TOKEN: ${CONVERTER_TOKEN}
+    labels:
+      - "com.centurylinklabs.watchtower.enable=true"
+    deploy:
+      resources:
+        limits:
+          cpus: "1.0"
+          memory: 1g
+
+networks:
+  postconvert-net:
+    name: postconvert-net