feat: add global error boundary and centralized exception handling

[nancysangani]

Closes #793

Implements a centralized error handling mechanism across the Flask application.

Changes:

• errors/handlers.py — all HTTP error handlers (400, 403, 404, 405, 429, 500) + unhandled exception catch-all, with HTML/JSON content negotiation for API routes
• utils/error_logger.py — secure server-side logging with correlation IDs; no stack traces ever reach the client
• app.py — replaced inline handlers with register_error_handlers(app)
• templates/400.html, templates/429.html — missing error pages added
• tests/test_error_handling.py — 30 tests covering all handlers, JSON envelope shape, security header preservation, and unhandled exception catch

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[nancysangani]

Hi @komalharshita, Please review the PR when you get a chance. Thanks!