[CLIENT-API-01] Implement client management API

[Krisor1986]

Goal

Provide the backend required by the client views.

Tasks

• Add client CRUD and search endpoints
• Add multiple client addresses
• Add notes, attachments and activity history
• Add next actions
• Return related leads, jobs and appointments

Acceptance criteria

• The client UI can operate without mock data
• Related records are returned consistently
• All queries enforce company isolation

Dependencies

• [DB-01]
• [MODEL-02]