Add persistence layer

No database/persistence exists yet. Add a persistence layer (choose DB + ORM/driver) behind the services/ layer, keeping api/ and mcp/ adapters unchanged.

The issue asks to add a database and ORM behind the services/ layer. First, examine the existing service layer to understand the data models and interfaces. Then, choose a database (e.g., PostgreSQL, SQLite) and an ORM (e.g., SQLAlchemy, Tortoise-ORM). The work involves designing the schema, integrating the ORM, and ensuring the API and MCP adapters remain unchanged. Done when the services can persist and retrieve data through the new layer.

Written by the indexing model from the issue text.