Migrate existing TOML configs to database service configs

## Overview

Migrate existing server config values (e.g., `hide_agents`) from TOML to database with backward compatibility.

## Tasks

### Config Migration

- Identify configs to migrate: `hide_agents`, etc.
- Update `ManagerConfigProvider` to check DB first, fallback to TOML

### Backward Compatibility

- If DB config not found, use TOML value
- Log warning when using TOML fallback
- Document migration path for operators

### Testing

- Unit tests for config resolution priority
- Integration tests for fallback behavior

## Files to Modify

- `src/ai/backend/manager/config/provider.py`
- `src/ai/backend/manager/repositories/image/repository.py` (uses hide_agents)
- Other files using migrated configs

JIRA Issue: BA-3763