[FEATURE] Add campaign updates / creator posts endpoint #566

Category: backend
Project area: backend/src/

Implementation:

• POST /api/campaigns/:id/updates — creator posts text update to backers.
• GET /api/campaigns/:id/updates — returns list of updates.
• Update stored with: creator_address, content, created_at.
• Webhook/email sent to all contributors on new update.

Acceptance Criteria:

• Only campaign creator can post updates.
• Update content max 2000 characters.
• Contributors notified via configured channels.