feat: add credential server for secure API key storage

Implements a credential server that stores API keys server-side,
eliminating the need to store credentials in the browser.

Key changes:
- New credential server with SQLite storage and encrypted credentials
- Setup page for API key entry (/setup)
- Proxy endpoint for API requests (/proxy)
- HTTP-only cookies for session management
- ProxiedAnthropicProvider for routing through credential server
- INIT command for worker to configure credential server URL
- Docker configuration for credential server deployment

Usage:
- Deploy credential server with CREDENTIAL_ENCRYPTION_SECRET env var
- Initialize ai-delegate with credentialServerUrl option
- Users visit /setup to enter their API key once
- Subsequent requests use session cookies automatically

Closes #2

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>