feat: version control engine — push, history, content-addressable deduplication

[Ali7040]

Implement VersioningService:

• POST /prompts/:id/versions — push new version
  • Compute SHA-256 hash of content (first 16 chars)
  • Reject if identical hash already exists for this prompt
  • Auto-increment version tag: v1.0.0 → v1.0.1
  • Store parentId to build the version tree
• GET /prompts/:id/versions — list all versions with author + eval count
• GET /prompts/:id/versions/:tag — get specific version by tag

Acceptance criteria:

• Pushing identical content twice returns 409 Conflict with the existing version tag
• Version tree is traversable via parentId