Centralized service configuration in database

## Overview

Move service-level behavioral configurations from server config files (TOML) to database for centralized management.

## Design

- Manager reads service configs from DB
- Manager applies configs when processing requests
- No config distribution to Agent/Storage Proxy needed

## Components

1. Database table `service_configs`
1. Repository with caching
1. Admin GraphQL API
1. Migration of existing TOML configs (e.g., `hide_agents`)

JIRA Issue: BA-3759