Edit Issue - .issue.db

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Description

Implement POST /api/v1/admin/users endpoint to create new users.

**API Specification:**
- Endpoint: POST /api/v1/admin/users
- Permission Required: manage_users
- Request Body: {email, display_name, roles[], is_active, metadata{}}
- Response: Created user object with user_id

**Requirements:**
- Validate email format and uniqueness within tenant
- Support assigning multiple roles on creation
- Send welcome email (optional flag)
- Auto-generate temporary password or send OAuth invite
- Log audit event for user creation
- Return 409 if email exists, 400 for validation errors

**Database:**
- Create users table if not exists (migration script)
- Store: user_id (UUID), tenant_id, email, display_name, is_active, created_at, created_by, metadata (JSONB)

**Testing:**
- Test with valid data
- Test duplicate email handling
- Test permission enforcement
- Test multi-role assignment

**IMPORTANT:** Implement with ultra attention to details and correctness.
**BEFORE CLOSING:** Take screenshot of API response in action. Add comment summarizing implementation.

Due Date (YYYY-MM-DD)

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Edit Issue #37