feat(setup): T05 configure pytest with coverage

- Create pytest.ini with:
  - Test discovery configuration (testpaths, python_files)
  - Asyncio mode settings
  - Coverage configuration (>=90% requirement)
  - Custom markers (unit, integration, e2e, slow)
- Update conftest.py with:
  - pytest_asyncio plugin
  - Shared fixtures (project_root, src_path, temp_dir, mock_env_vars)
  - Path configuration for imports
- Add test_pytest_config.py with 12 unit tests
- All tests passing (12/12)

Refs: T05

Completes setup phase T01-T05

pytest.ini

@@ -0,0 +1,32 @@
+[pytest]
+# Test discovery settings
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+
+# Asyncio settings
+asyncio_mode = auto
+asyncio_default_fixture_loop_scope = function
+
+# Coverage settings
+addopts = 
+    -v
+    --strict-markers
+    --tb=short
+    --cov=src/openrouter_monitor
+    --cov-report=term-missing
+    --cov-report=html:htmlcov
+    --cov-fail-under=90
+
+# Markers
+testmarkers =
+    unit: Unit tests (no external dependencies)
+    integration: Integration tests (with mocked dependencies)
+    e2e: End-to-end tests (full workflow)
+    slow: Slow tests (skip in quick mode)
+
+# Filter warnings
+filterwarnings =
+    ignore::DeprecationWarning:passlib.*
+    ignore::UserWarning