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")

# Check account status (cached)
status = client.get_api_status()
print(status["quota"]["remaining"])  # API credits left
print(status["is_pro"])              # Pro subscription
print(status["level"])               # User level (User, BountyHunter, etc.)

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

get_api_status() Response

Uses the new /api/user/info endpoint (requires frontend update):

{
  "authenticated": true,
  "username": "myuser",
  "email": "user@example.com",
  "level": "BountyHunter",
  "is_pro": true,
  "quota": {"total": 30000, "remaining": 29500, "used": 500},
  "features": ["search", "host_lookup", "bulk_export", "pro_plugins"],
  "created": 1234567890
}

Why AsyncClient?

• Required for MCP servers (Model Context Protocol)
• Works with FastAPI, aiohttp, and other async frameworks
• Rate limit retry with exponential backoff (1s, 2s, 4s on 429)
• Streaming support for large bulk exports

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