refactor: consolidate transaction management and remove legacy modules

- Remove unused context.py module (replaced by request.state)
- Remove provider_cache.py (no longer needed)
- Unify environment loading in config/settings.py instead of __init__.py
- Add deprecation warning for get_async_db() (consolidating on sync Session)
- Enhance database.py documentation with comprehensive transaction strategy
- Simplify audit logging to reuse request-level Session (no separate connections)
- Extract UsageService._build_usage_params() helper to reduce code duplication
- Update model and user cache implementations with refined transaction handling
- Remove unnecessary sessionmaker from pipeline
- Clean up audit service exception handling

src/__init__.py

@@ -3,10 +3,8 @@
 A proxy server that enables AI models to work with multiple API providers.
 """
 
-from dotenv import load_dotenv
-
-# Load environment variables from .env file
-load_dotenv()
+# 注意: dotenv 加载已统一移至 src/config/settings.py
+# 不要在此处重复加载
 
 try:
     from src._version import __version__