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

tests/conftest.py

@@ -0,0 +1,50 @@
+"""Pytest configuration and fixtures.
+
+This module contains shared fixtures and configuration for all tests.
+"""
+import sys
+import os
+import pytest
+import pytest_asyncio
+
+# Add src to path for importing in tests
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..', 'src'))
+
+# Markers for test organization
+pytest_plugins = ['pytest_asyncio']
+
+
+def pytest_configure(config):
+    """Configure pytest with custom markers."""
+    config.addinivalue_line("markers", "unit: Unit tests (no external dependencies)")
+    config.addinivalue_line("markers", "integration: Integration tests (with mocked dependencies)")
+    config.addinivalue_line("markers", "e2e: End-to-end tests (full workflow)")
+    config.addinivalue_line("markers", "slow: Slow tests (skip in quick mode)")
+
+
+@pytest.fixture(scope='session')
+def project_root():
+    """Return the project root directory."""
+    return os.path.dirname(os.path.dirname(__file__))
+
+
+@pytest.fixture(scope='session')
+def src_path(project_root):
+    """Return the src directory path."""
+    return os.path.join(project_root, 'src')
+
+
+@pytest.fixture
+def temp_dir(tmp_path):
+    """Provide a temporary directory for tests."""
+    return tmp_path
+
+
+@pytest.fixture
+def mock_env_vars(monkeypatch):
+    """Set up mock environment variables for testing."""
+    monkeypatch.setenv('SECRET_KEY', 'test-secret-key-min-32-characters-long')
+    monkeypatch.setenv('ENCRYPTION_KEY', 'test-32-byte-encryption-key!!')
+    monkeypatch.setenv('DATABASE_URL', 'sqlite:///./test.db')
+    monkeypatch.setenv('DEBUG', 'true')
+    monkeypatch.setenv('LOG_LEVEL', 'DEBUG')