Use importlib.metadata.version() for __version__

[dannywillems]

Problem

client.py hardcodes __VERSION__ = "0.1.9" while pyproject.toml has version = "0.1.10". This version drift is a bug.

Solution

• Use importlib.metadata.version("leakix") in __init__.py as the single source of truth
• Remove the hardcoded __VERSION__ from client.py
• Update the User-Agent header in Client to reference the package-level version

Standards

• PEP 396 / PEP 566: single source of truth for version in pyproject.toml