Create Spring Boot starter module

Provide auto-configured Spring Boot integration.

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## ✅ Challenge Spec

## 🧩 Problem
What adoption barrier does this solve (deployment, API usability, integration friction, etc.)?

## 🎯 Goal
Make ExtractPDF4J easier to use in real services/pipelines (Spring Boot/Kafka/Docker/etc.).

## ✅ Acceptance Criteria
- Example builds and runs successfully with clear instructions
- Provide a minimal runnable demo (README + commands)
- Uses the latest stable ExtractPDF4J version
- Document prerequisites (native libs, env vars, Docker notes)
- Add a basic smoke test where feasible

## 🧪 Validation
- Steps to run locally (copy/paste)
- Expected output examples (JSON/CSV/table)
- Optional: Docker compose / curl examples

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**Challenge Note:** This issue is part of the ExtractPDF4J Global Build Challenge 2026.
To participate: fork the repo → create a branch → submit a PR referencing this issue → Star the repo.