feat: rewrite backend in Rust (axum)

Replace the Node/Express backend with a Rust implementation that keeps
the same REST API, MongoDB collections and Redis cache keys, so existing
users, subscriptions and progress keep working.

- axum + mongodb + redis, bcrypt and HS256 JWT compatible with old data
- RSS parsing, iTunes discovery/trending, background sync (1h / 4h)
- Docker image now builds a static Rust binary on debian-slim
- CI runs cargo tests before publishing the image
- compose uses pull_policy: always so a restart picks up the new image

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -30,12 +30,14 @@ services:
 
   app:
     image: ghcr.io/r0m1k3/podcastic:latest
+    # Always fetch the newest image when the stack (re)starts
+    pull_policy: always
     container_name: podcastic-app
     restart: unless-stopped
     ports:
       - "${PORT:-3579}:3579"
     environment:
-      NODE_ENV: production
+      RUST_LOG: podcastic=info,tower_http=warn
       PORT: 3579
       MONGODB_URI: mongodb://mongodb:27017/podcastic
       REDIS_URL: redis://redis:6379