fix: enforce email or webhook_url on notification preferences

[Akintobidavid]

Overview

This PR fixes the notification-service validation gap in PUT /preferences/:address. The handler now checks the resulting preference and returns 400 when neither email nor webhook_url is set. Empty bodies and bodies containing only { "enabled": true } no longer silently create preferences that cannot notify anyone.

Related Issue

Changes

🔔 Notification Preference Validation

• [MODIFY] notification-service/src/api.ts
  • Validate that the resulting preference has at least one notification channel.
  • Reject with 400 when both email and webhook_url are absent, null, or empty string.
  • Return a clear error body: { "error": "At least one of email or webhook_url must be provided." }
  • Preserve existing preferences that already have email or webhook_url set when an update omits both fields.
  • Allow updates such as { "enabled": false } on a preference that already has one channel configured.

Verification Results

npm test -- notification-service/src/api.test.ts
✅ 12/12 passed

Manual API checks:
✅ PUT /preferences/:address with {} → 400
✅ PUT /preferences/:address with { "enabled": true } → 400
✅ PUT /preferences/:address with { "email": "[email protected]" } → 200
✅ PUT /preferences/:address with { "webhook_url": "https://example.com/hook" } → 200
✅ PUT existing preference with email set, body { "enabled": false } → 200

Acceptance Criteria	Status
PUT /preferences/:address with neither email nor webhook_url (and no pre-existing preference) returns 400 with a clear error message	✅ Verified with empty body and enabled-only body
Updating an existing preference that already has one channel set, without supplying either field again, is still allowed	✅ Verified with existing email preference + { "enabled": false }

Closes #338

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