Add FastAPI REST API for pgmq

[javadghane]

Summary

• add a FastAPI application that exposes queue management, message, batch, and metrics endpoints for pgmq
• expose a lazy create_app helper without introducing a hard FastAPI import at package import time
• document the new API optional dependency group and add unit coverage for the API surface using a fake queue

Testing

• PYTHONPATH=src PYENV_VERSION=3.10.17 pytest tests/test_api.py (skipped: FastAPI not installed in environment)

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https://chatgpt.com/codex/tasks/task_e_68fb8b86ba4083299749cee8915d8389