Feat: Add AsyncClient, search API, and Pro detection

[Chocapikk]

Summary

This PR enhances the LeakIX Python client with modern async support and improved developer experience.

New Features

• AsyncClient: Full async/await support using httpx for async applications (MCP servers, FastAPI, etc.)
• Simple search() API: Intuitive method accepting raw query strings like the website
• Account status: get_api_status() and is_pro() methods using /api/user/info endpoint (cached)
• Rate limit handling: Automatic retry with exponential backoff on 429 responses
• Streaming bulk export: Memory-efficient bulk_export_stream() generator

API Examples

# Sync client
from leakix import Client
client = Client(api_key="...")
results = client.search("+plugin:GitConfigHttpPlugin", scope="leak")

# Async client
from leakix import AsyncClient
async with AsyncClient(api_key="...") as client:
    results = await client.search("+country:FR +port:22", scope="service")

Changes

• Lower Python requirement from 3.13 to 3.10
• Add leakix/async_client.py with full async implementation
• Add search(), get_domain(), get_api_status(), is_pro() to both clients
• Add bulk_export_stream() for memory-efficient streaming
• Export AsyncClient from package root
• Version bump to 0.2.0

Recreated from #27 as an internal branch for CI.