feat: implement Cross-Module Unified Notification & Alert Registry

[SatyamPandey-07]

1. Core Registry & Models
   backend/models/alert.py
   : Created a high-gravity
   Alert
   model with support for priority levels (CRITICAL, HIGH, MEDIUM, LOW), categories, and multi-channel delivery tracking (WebSockets, Email, SMS).
   backend/services/alert_registry.py
   : A centralized dispatcher that evaluates user preferences, aggregates similar alerts to prevent "notification fatigue," and handles priority-based queuing.

2. Real-Time Infrastructure
   backend/sockets/alert_socket.py
   : Dedicated WebSocket handlers for private and global alert broadcasting.
   js/alert_manager.js
   : A robust frontend manager that handles incoming socket payloads, plays alert sounds for critical events, and manages the local unread state.

3. Redesigned UI/UX
   navbar.html
   : Integrated a modern "Alert Hub" dropdown with live badges and CSS-driven toast notifications for real-time visibility.
   notifications.html
   : A new, dedicated management center where users can filter alerts by category/priority and configure their cross-channel delivery preferences.

4. Service Integration & Maintenance
   backend/services/weather_service.py
   : Automatically triggers "Extreme Heat" and "Heavy Rainfall" CRITICAL alerts.
   backend/services/market_service.py
   : Refactored to send HIGH priority "Profit Opportunity" alerts when watchlist targets are reached.
   backend/tasks/alert_cleanup.py
   : Celery tasks for automated archival of read alerts and purging of expired registry entries.

closes #1461

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[github-actions]

🎉 Congrats @SatyamPandey-07 on getting your PR merged! 🙌
Thanks for the contribution — looking forward to more from you 🚀