docs(README): add social preview banner and shields.io badges

Author: laurenz-glueck

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+<p align="center">
+  <img src=".github/social-preview.png" alt="Fakturia MCP - Connect AI to Billing" width="640" />
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 # Fakturia API MCP
 
+[![npm version](https://img.shields.io/npm/v/@edjufy/fakturia-mcp)](https://www.npmjs.com/package/@edjufy/fakturia-mcp)
+[![license](https://img.shields.io/npm/l/@edjufy/fakturia-mcp)](LICENSE)
+[![node](https://img.shields.io/node/v/@edjufy/fakturia-mcp)](package.json)
+[![TypeScript](https://img.shields.io/badge/TypeScript-5.8-blue?logo=typescript&logoColor=white)](https://www.typescriptlang.org/)
+[![MCP Tools](https://img.shields.io/badge/MCP_Tools-123-brightgreen)](https://modelcontextprotocol.io/)
+
 An unofficial [MCP (Model Context Protocol)](https://modelcontextprotocol.io/) server that provides full access to the [Fakturia](https://www.fakturia.de) billing and subscription management API. Connect your AI assistant to Fakturia and manage customers, contracts, invoices, subscriptions, and more through natural language.
 
 > **Disclaimer:** This is an unofficial, community-built project. It is not affiliated with, endorsed by, or supported by [Luminea IT Services GmbH](https://www.fakturia.de/impressum), the company behind Fakturia. "Fakturia" is a product of Luminea IT Services GmbH. The included OpenAPI spec in `docs/api/` was obtained from Fakturia's public API documentation endpoint.