feat: Add BC Notify and BC Notify Housing services.

[pwei1018]

The changes are the addition of BC Notify as a notification provider (with feature flags for dynamic selection), implementation of SSRF protections for file downloads, improved logging for database operations, and updates to email validation and its tests.

Notification Provider Enhancements:

• Added BC Notify and BC Notify Housing as notification providers

Security Improvements:

• Replaced requests with httpx for outbound HTTP requests and implemented SSRF protections in download_file, restricting downloads to public http/https URLs and enforcing size/time limits.

Error Handling and Logging:

• Improved error handling in database operations for callbacks and safe list management, adding structured logging for failures.

API and Testing Updates:

• Updated the email validation endpoint to require authentication and refactored preflight handling for CORS. Tests were updated to inject valid tokens and to mock httpx instead of requests.

Other Improvements:

• Minor code cleanups and improved documentation/comments throughout the codebase.