Implement dependency injection system

- Enhanced existing src/server/utils/dependencies.py with optional SQLAlchemy import
- Added comprehensive unit tests in tests/unit/test_dependencies.py
- Created pytest configuration with asyncio support
- Implemented SeriesApp singleton dependency with proper error handling
- Added placeholders for database session and authentication dependencies
- Updated infrastructure.md with dependency injection documentation
- Completed dependency injection task from instructions.md

Features implemented:
- SeriesApp dependency with lazy initialization and singleton pattern
- Configuration validation for anime directory
- Comprehensive error handling for initialization failures
- Common query parameters for pagination
- Placeholder dependencies for future authentication and database features
- 18 passing unit tests covering all dependency injection scenarios

src/server/utils/dependencies.py

@@ -9,7 +9,11 @@ from typing import AsyncGenerator, Optional
 
 from fastapi import Depends, HTTPException, status
 from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
-from sqlalchemy.ext.asyncio import AsyncSession
+
+try:
+    from sqlalchemy.ext.asyncio import AsyncSession
+except ImportError:
+    AsyncSession = None
 
 from src.config.settings import settings
 from src.core.SeriesApp import SeriesApp
@@ -59,7 +63,7 @@ def reset_series_app() -> None:
     _series_app = None
 
 
-async def get_database_session() -> AsyncGenerator[AsyncSession, None]:
+async def get_database_session() -> AsyncGenerator[Optional[object], None]:
     """
     Dependency to get database session.
     
@@ -144,7 +148,7 @@ class CommonQueryParams:
 
 
 def common_parameters(
-    skip: int = 0, 
+    skip: int = 0,
     limit: int = 100
 ) -> CommonQueryParams:
     """
@@ -177,4 +181,4 @@ async def log_request_dependency():
     
     TODO: Implement request logging logic
     """
-    pass
+    pass