Add authentication feature with session management

- Implemented authentication logic in src/server/auth.js, including session token creation and verification.
- Integrated authentication routes for login, logout, and user session retrieval in the Fastify server.
- Updated docker-compose.yml to include optional authentication environment variables.
- Added @fastify/cookie dependency for cookie management in package.json and package-lock.json.
- Enhanced the frontend with login form and session handling in App.svelte, including UI updates for authenticated states.
- Styled authentication components in app.css for better user experience.

docker-compose.yml

@@ -19,6 +19,11 @@ services:
       # Each title is eligible again after this many hours (daily).
       WATCH_TITLE_INTERVAL_HOURS: "24"
       FLARESOLVERR_URL: http://flaresolverr:8191/v1
+      # Shared login (optional). When AUTH_PASSWORD is set, the UI and /api require a session.
+      # AUTH_USERNAME: admin
+      # AUTH_PASSWORD: change-me
+      # SESSION_SECRET: long-random-string-at-least-32-chars
+      # SESSION_TTL_HOURS: "168"
     volumes:
       - ./data:/app/data
     depends_on: