Route notification images through API content gateway

[raymondjacobson]

Summary

• default notification image URLs to https://api.audius.co instead of a hard-coded creator node
• stop using stored creator_node_endpoint values for track and playlist email images
• add NOTIFICATIONS_CONTENT_NODE_ENDPOINT as a notification-specific override
• update notification mapping expectations to use the API content gateway

Dependency

• depends on Add content asset redirect gateway api#985 being deployed before this is merged/released

Testing

• npm test -w @pedalboard/notifications -- --runInBand src/__tests__/contentNodeEnv.test.ts
• npm run typecheck -w @pedalboard/notifications
• git diff --check

Notes

• The broader notifications mapping/email Jest command still needs local discovery DB env and fails before assertions in populateDB.ts.
• Remaining creatornode2.audius.co references in archiver/backfill use non-/content APIs and were left for a separate follow-up.