refactor: combine credential server with ai-delegate server

Merged credential server functionality into the main ai-delegate server:
- Server now handles both worker file serving and credential management
- Credentials enabled when CREDENTIAL_ENCRYPTION_SECRET env var is set
- Updated Dockerfile with SQLite build deps and /data volume
- Single container deployment instead of separate credential server

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