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>

dev.sh

@@ -42,7 +42,7 @@ echo ""
 # Start backend in background
 echo "→ Backend starting on port 5000..."
 cd backend
-npm run dev &
+PORT=5000 PUBLIC_DIR=../frontend/dist cargo run &
 BACKEND_PID=$!
 cd ..