[RFC-0008] Implement error handling

[copyleftdev]

TASK-009: Implement error handling

RFC: RFC-0008 - API Layer
Component: Error Handling
Estimate: s

Description

Create ApiError enum with variants. Implement IntoResponse for consistent JSON error format. Handle redirect to leader.

Acceptance Criteria

• AC-004: Write requests on follower return redirect to leader
  • Verification: POST to follower returns 307 with leader location

Dependencies

Depends on: TASK-001