Enable customizable topic subscriptions and real-time alerts

Description:
Currently SynthoraAI sends a single daily newsletter covering all articles. To improve relevance and engagement, allow users to subscribe to specific topics, keywords or sources and receive real-time alerts (via email and/or in-app/web notifications) when new matching articles are ingested.

Acceptance Criteria:

1. Users can select one or more topics, keywords, or sources from the frontend “Preferences” or “Subscriptions” page.
2. Subscriptions are persisted in the user profile (MongoDB).
3. When the crawler/backend ingests a new article that matches a user’s subscription criteria, the system immediately enqueues a notification job.
4. Notifications support both:
   • Email: via existing Resend integration
   • Web Push / In-app: via a new notifications endpoint and WebSocket or polling fallback
5. Users can manage (add/remove) their subscriptions and toggle real-time vs. daily digest modes.
6. All subscription and notification APIs are documented in Swagger.

Tasks:

• Add Subscription schema in Mongoose (fields: userId, topics, keywords, sources, mode)
• Build frontend “Subscriptions” UI: multi-select topics, keyword input, source checkboxes, mode toggle
• Create backend endpoints:
  • GET /api/subscriptions
  • POST /api/subscriptions
  • PUT /api/subscriptions/:id
  • DELETE /api/subscriptions/:id
• Extend crawler/backend ingestion pipeline to evaluate article metadata against active subscriptions and enqueue notification jobs
• Implement notification service:
  • Email notifications via Resend
  • Web Push / in-app notifications via Socket.io or polling
• Add notification preferences to user settings and persist choices
• Write Playwright and Jest tests for subscription flows, ingestion triggers, and notification delivery
• Update Swagger docs with new subscription & notification endpoints

Estimated Effort: ~2 sprints