docs: rewrite CONFIGURATION.md for DB-only config model

- Remove all YAML configuration examples
- Document configuration via Admin UI and Management API instead
- Describe settings conceptually (tables of options, not code blocks)
- Reorganize sections: Environment → UI/API → Providers → Models → Keys → Quotas → Cooldowns → MCP → Vision → Pricing → Encryption → Failover
- Add Management API endpoint reference table
- Clearer tables for quota types, selector strategies, pricing sources

Author: mcowger