feat(followers): implement CRDT-based follower count to eliminate los…

[Alaka-ibr]

feat(followers): implement CRDT-based follower count to eliminate lost updates (#757)

Summary

Closes #757

Under concurrent follow and unfollow bursts for the same creator, direct integer increment/decrement updates suffer from lost updates and race conditions. This PR replaces the simple counter with a G-Counter CRDT structure per node, computing the true count as the sum of all increments minus decrements across all nodes at read time.

Changes

1. Database Schema (prisma/schema/follower.prisma)

• FollowerCounterShard: creatorWallet, nodeId, increments (BigInt), decrements (BigInt), updatedAt. Unique key: (creatorWallet, nodeId).
• FollowEvent: followerWallet, creatorWallet, direction (FOLLOW | UNFOLLOW), createdAt. Unique key: (followerWallet, creatorWallet).

2. Node Identification (src/config.schema.ts)

• Added NODE_ID configuration parameter (defaults to node-local).

3. Atomic Follow/Unfollow & Count Resolution (src/modules/followers/follower.service.ts)

• Follow & Unfollow: Idempotent event check. Performs database-level atomic shard increment (UPDATE ... SET increments = increments + 1) for local NODE_ID.
• Count Resolution: getFollowerCount sums increments and decrements across all shards for creatorWallet, returns $\max(0, \sum \text{inc} - \sum \text{dec})$. Cached in Redis with a 10-second TTL (invalidated on write).
• Atomic Compaction: compactShardsForCreator merges all shards for a creator into a single canonical shard in a single transaction (skips creators updated in the last 5 minutes).

4. Endpoints & Nightly Job (src/modules/followers/follower.controllers.ts, src/jobs/follower-shard-compaction.job.ts)

• POST /api/v1/followers/:creatorWallet/follow
• POST /api/v1/followers/:creatorWallet/unfollow
• GET /api/v1/followers/:creatorWallet/count
• Nightly compaction job runNightlyFollowerShardCompaction.

Test Coverage

Unit tests in src/modules/followers/follower.service.test.ts (5 passed):

• 100 concurrent follows from different wallets produces count of exactly 100 with zero lost updates.
• 50 concurrent follows and 30 concurrent unfollows produces net count of 20.
• Double-follow from the same wallet is idempotent and increments count only once.
• Resolves count correctly across multiple distinct node shards.
• Nightly compaction merges shards atomically while preserving identical resolved count.