Design and implement stats query API for frontend integration

[NirrWorks]

We need a backend API layer for OSMSG so the frontend can request stats directly from PostgreSQL instead of relying on CLI/file outputs.
This issue focuses on designing and implementing the initial FastAPI endpoints for stats retrieval.

[NirrWorks]

Update: I have opened PR #37 for the initial API implementation.

The approach changed slightly from the original FastAPI idea in this issue. Based on the latest discussion, the API is implemented with Litestar under osmsg/api.

Current scope in the PR:

• GET /health
• GET /api/v1/user-stats
• PostgreSQL access through asyncpg
• DATABASE_URL loaded from environment variable
• API dependencies placed under the explicit api dependency group
• .env.example added
• README instructions added for inserting data and running the API
• API tests added with request/response assertions

The API has also been tested with hosted Neon Postgres and deployed on Render for demo.