fix(ci): standardize Python dependency management across all jobs

- Add requirements.txt with core Python dependencies
- Replace --target installation with setup-python@v4 (official action)
- All jobs now use cache: 'pip' for consistent caching
- Explicit 'pip install -r requirements.txt' or specific packages
- Fixes 'No module named pytest' in ci-storage job
- Fixes 'No file matched to requirements.txt' in Setup Python step
- All jobs: pyyaml, requests, openpyxl, pytest, psycopg installed globally
- Removes PYTHONPATH env vars (no longer needed with proper setup-python)

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

requirements.txt

@@ -0,0 +1,25 @@
+# QuantEngine v0.2 - Python Dependencies
+# CI/CD validation and data collection tools
+
+# Core dependencies
+pyyaml>=6.0
+requests>=2.28.0
+python-dotenv>=0.21.0
+
+# Data processing
+openpyxl>=3.9.0
+pandas>=1.5.0
+numpy>=1.23.0
+
+# Database
+psycopg[binary]>=3.1.0
+
+# Testing & validation
+pytest>=7.2.0
+pytest-asyncio>=0.20.0
+
+# Async
+aiohttp>=3.8.0
+
+# Utilities
+click>=8.1.0