feat: implement feed url updated notification

[davidgamez]

Summary:

This PR introduces the feed URL update notification. The URL update notification has two variants: URL replacement and feed redirect. Added helpers to emit notifications, rate limiting, and Brevo integration.

Expected behavior:

• When a URL is replaced on automated imports/operations API, a feed.url_updated => feed_redirected event is logged into the notification_event table
• When a feed is redirected using the MD catalog import/operations API, a feed.url_updated => url_replaced event is logged into the notification_event table.
• Events are sent every day on schedule for daily cadence and on Monday for Weekly cadence

Testing tips:

[Internal team]
For integrated tests, use DEV environment. As the notification endpoints are not implemented, notification-related entities need to be created via SQL.

• Subscribe a user to a notification by inserting a row in notification_subscription table; replace with the target user id(REPLACE_ME_WITH_USER_ID)

 INSERT INTO notification_subscription (id, user_id, notification_type_id, cadence, digest, active, active_since)
 VALUES (gen_random_uuid()::text, REPLACE_ME_WITH_USER_ID, 'feed.url_updated',
'weekly', false, true, now() - interval '1 day');

• Simulate a notification_event:

WITH ev AS ( INSERT INTO notification_event (id, notification_type_id, event_subtype, source, payload, created_at) VALUES (gen_random_uuid()::text, 'feed.url_updated', 'url_replaced', 'manual_test',
  '{"old_url":"https://old/feed.zip","new_url":"https://new/feed.zip"}'::jsonb, now()) RETURNING id)
 INSERT INTO notification_event_feed (id, notification_event_id, feed_stable_id, role)
 SELECT gen_random_uuid()::text, ev.id, 'mdb-10', 'subject' FROM ev;

• Call the notifications_dispatch_plan task from retool(DEV)
• Verify that the user receives a raw email(not from a template)

From our AI friend

This pull request introduces several improvements and new features to the notification system and environment configuration for database update workflows. The main changes include the addition of a generic rate limiter utility, enhancements to notification event emission for feed updates and redirects, and the introduction of more flexible environment variable handling for user databases in CI/CD workflows.

Key changes:

Notification System Enhancements

• Introduced notification_constants.py, which defines namespaced string constants for notification types, event subtypes, cadences, statuses, sources, and feed roles, improving maintainability and reducing magic strings throughout the notification codebase.
• Added module-level documentation to __init__.py in the shared.notifications package, clarifying the available exports and their purposes.

Notification Event Emission in Database Population Scripts

• Updated populate_db_gtfs.py and populate_db_gbfs.py to emit notification events when a feed's producer URL is replaced or when a feed is redirected. This enables downstream systems to react to important feed changes and improves auditability. [1] [2] [3] [4] [5]

Utility Improvements

• Added a new, reusable, thread-safe token-bucket rate limiter (rate_limiter.py) with a process-wide registry, allowing consistent and testable rate limiting for outbound API calls across the codebase.

CI/CD Workflow Improvements

• Enhanced GitHub Actions workflows (db-update-content.yml, catalog-update.yml) to support a separate USER_DB_ENVIRONMENT variable, allowing the user database environment to be configured independently from the main DB environment. This is particularly useful for development setups that share infrastructure. [1] [2] [3]

These changes collectively improve the robustness, configurability, and observability of both the notification system and deployment workflows.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[davidgamez]

[out-of-scope]: Configurations should be more dynamic, avoiding the need for redeployment when they are changed.

[davidgamez]

[out-of-scope]: internationalization messages