Auth: Role management endpoints (members, roles)

[EricGrill]

Auth System — Step 7 (Backend)

Manage users and roles within a farm.

Endpoints:

• GET /api/farms/:farmId/members — list users with roles on this farm
• POST /api/farms/:farmId/members/:userId/role — assign/change role
• DELETE /api/farms/:farmId/members/:userId — remove user from farm

Access: admin or farm_owner for the specific farm.

Implementation:

• Members list with user name, email/phone, role, joined date
• Role change with hierarchy validation
• Remove user (deletes user_farm_roles row, not the user account)
• Cannot remove last farm_owner from a farm

Depends on: Auth middleware applied to routes
Design doc: docs/AUTH.md