Fix datetime deprecation warnings in Phase 8 modules

- Update all datetime.utcnow() to datetime.now(UTC) for Python 3.12+ compatibility
- Update all datetime.utcfromtimestamp() to datetime.fromtimestamp(..., UTC)
- Fix dataclass default_factory to use lambda: datetime.now(UTC)
- Update auth, audit, billing, and realtime modules
- Add UTC import from datetime module
- Update pytest configuration to include pytest-asyncio
- All 28 Phase 8 enterprise tests pass with no warnings

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

pyproject.toml

@@ -14,9 +14,10 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
-test = ["pytest>=8"]
+test = ["pytest>=8", "pytest-asyncio>=0.23"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 pythonpath = ["."]
 addopts = "-p no:cacheprovider"
+asyncio_mode = "auto"