Bonus: Implement webhook notifications

[copyleftdev]

Description

Add webhook support for async job completion notifications.

Tasks

• Accept webhook_url in requests
• Store webhook configs in database
• Implement HMAC signature generation
• Send webhook on analysis completion
• Add retry logic with exponential backoff
• Implement replay protection

Documentation

• See docs/02-api-specification.md Section 6 for webhook spec
• See docs/03-system-architecture.md for notification layer

Acceptance Criteria

• Webhooks are sent on completion
• HMAC signatures are valid
• Retries work correctly