[CHANNEL-01] Implement omnichannel provider adapters

[Krisor1986]

Goal

Create a common integration layer for ClientFlow communication channels.

Tasks

• Define a provider adapter interface
• Implement the web chat adapter
• Prepare email adapter configuration
• Document WhatsApp and Instagram webhook contracts
• Normalize inbound and outbound messages
• Add retries and delivery status

Acceptance criteria

• The inbox does not depend on provider-specific message formats
• Web chat works end to end
• External provider requirements are documented

Dependencies

• [INBOX-01]