feat(setup): T02 initialize virtual environment and gitignore

- Create comprehensive .gitignore with Python, venv, DB exclusions
- Add test_virtual_env_setup.py with 6 unit tests
- Verify Python 3.13.5 compatibility (>= 3.11 required)
- All tests passing (6/6)

Refs: T02

.gitignore

@@ -0,0 +1,69 @@
+# ===========================================
+# OpenRouter API Key Monitor - .gitignore
+# ===========================================
+
+# Virtual environments
+.venv/
+venv/
+ENV/
+env/
+
+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+
+# Database files
+*.db
+*.sqlite
+*.sqlite3
+*.db-journal
+
+# Environment variables
+.env
+.env.local
+.env.*.local
+!.env.example
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+.DS_Store
+
+# Testing
+.pytest_cache/
+.coverage
+htmlcov/
+.tox/
+.nox/
+
+# Logs
+*.log
+logs/
+
+# Data directory (for local development)
+data/
+
+# Alembic
+alembic/versions/*.py
+!alembic/versions/.gitkeep