feat: add follow endpoint with integration tests

[Emeka-12]

Summary

Add integration tests for the follow/unfollow endpoints to verify correct follower count incrementing and decrementing behavior. This also includes the implementation of the follow feature itself.

closes #697, closes #698, closes #699, closes #700

Changes

Schema

• Added Follow Prisma model with unique constraint on (followerAddress, creatorId)
• Added followersCount field to CreatorProfile model (default: 0)

Implementation

• Follow Service (follow.service.ts): Idempotent follow/unfollow logic with atomic transactions
• Follow Handlers (follow.handlers.ts): Express handlers for POST/DELETE endpoints
• Follow Routes (creator.routes.ts): Added routes with Stellar signature authentication

Integration Tests

• Follow increments count by 1: POST to follow endpoint returns 201 with followersCount: 1
• Double follow is idempotent: Second follow returns 200 with action: 'already_following', count unchanged
• Unfollow decrements count by 1: DELETE returns 200 with followersCount: 0
• Double unfollow is idempotent: Second unfollow returns 200 with action: 'not_following', count stays 0
• Unauthenticated follow returns 401: Request without wallet headers returns UNAUTHORIZED
• Non-existent creator returns 404: Follow attempt on missing creator returns NOT_FOUND

Endpoints Added

Method	Path	Description
POST	/api/v1/creators/:creatorId/follow	Follow a creator (idempotent)
DELETE	/api/v1/creators/:creatorId/follow	Unfollow a creator (idempotent)

Both endpoints require Stellar signature authentication (x-wallet-address, x-wallet-signature, x-timestamp headers).

Test Results

All 6 integration tests pass:

PASS src/modules/creator/follow.integration.test.ts
  POST/DELETE /api/v1/creators/:creatorId/follow — follower count
    ✓ increments follower count by 1 on follow
    ✓ does not increment count on double follow (idempotent)
    ✓ decrements follower count by 1 on unfollow
    ✓ does not decrement count below 0 on double unfollow (idempotent)
    ✓ returns 401 for unauthenticated follow request
    ✓ returns 404 when creator does not exist

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