Create service_configs table and repository

## Overview

Create database infrastructure for storing service configurations.

## Tasks

### Database Table

- Create `service_configs` table via alembic migration
- Schema: `id`, `key` (unique), `value` (JSONB), `description`, `created_at`, `updated_at`

### Repository

- `ServiceConfigRepository` with CRUD operations
- TTL-based caching for performance
- Methods: `get(key)`, `set(key, value)`, `delete(key)`, `list_all()`

### Seed Data

- Default configuration values for initial deployment

## Files to Create/Modify

- `src/ai/backend/manager/models/service_config/row.py`
- `src/ai/backend/manager/repositories/service_config/repository.py`
- Alembic migration file

JIRA Issue: BA-3760