feat(tasks): T55-T58 implement background tasks for OpenRouter sync

- T55: Setup APScheduler with AsyncIOScheduler and @scheduled_job decorator
- T56: Implement hourly usage stats sync from OpenRouter API
- T57: Implement daily API key validation job
- T58: Implement weekly cleanup job for old usage stats
- Add usage_stats_retention_days config option
- Integrate scheduler with FastAPI lifespan events
- Add 26 unit tests for scheduler, sync, and cleanup tasks
- Add apscheduler to requirements.txt

The background tasks now automatically:
- Sync usage stats every hour from OpenRouter
- Validate API keys daily at 2 AM UTC
- Clean up old data weekly on Sunday at 3 AM UTC

src/openrouter_monitor/main.py

@@ -2,6 +2,8 @@
 
 Main application entry point for OpenRouter API Key Monitor.
 """
+from contextlib import asynccontextmanager
+
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
@@ -11,15 +13,32 @@ from openrouter_monitor.routers import auth
 from openrouter_monitor.routers import public_api
 from openrouter_monitor.routers import stats
 from openrouter_monitor.routers import tokens
+from openrouter_monitor.tasks.scheduler import init_scheduler, shutdown_scheduler
 
 settings = get_settings()
 
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """Application lifespan manager.
+    
+    Handles startup and shutdown events including
+    scheduler initialization and cleanup.
+    """
+    # Startup
+    init_scheduler()
+    yield
+    # Shutdown
+    shutdown_scheduler()
+
+
 # Create FastAPI app
 app = FastAPI(
     title="OpenRouter API Key Monitor",
     description="Monitor and manage OpenRouter API keys",
     version="1.0.0",
     debug=settings.debug,
+    lifespan=lifespan,
 )
 
 # CORS middleware