fix: replace deprecated @app.on_event with lifespan handlers

Replace deprecated @app.on_event("startup") and @app.on_event("shutdown")
with the modern lifespan context manager pattern using asynccontextmanager.

This removes deprecation warnings from test output and uses the recommended
FastAPI approach for application lifecycle events.

Author: DerekRoberts

backend-py/src/main.py

@@ -1,5 +1,6 @@
 import time
 import uuid
+from contextlib import asynccontextmanager
 
 import structlog
 from fastapi import FastAPI, Request
@@ -25,10 +26,27 @@
     },
 ]
 
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """Lifespan context manager for startup and shutdown events."""
+    # Startup
+    logger.info(
+        "Application startup complete",
+        service="backend-py",
+        version="0.1.0",
+        api_prefix=api_prefix_v1,
+    )
+    yield
+    # Shutdown
+    logger.info("Application shutdown initiated")
+
+
 app = FastAPI(
     title=OpenAPIInfo["title"],
     version=OpenAPIInfo["version"],
     openapi_tags=tags_metadata,
+    lifespan=lifespan,
 )
 
 
@@ -147,20 +165,3 @@ async def root():
 
 
 app.include_router(user_router, prefix=api_prefix_v1 + "/user", tags=["User CRUD"])
-
-
-# Startup event
-@app.on_event("startup")
-async def startup_event():
-    logger.info(
-        "Application startup complete",
-        service="backend-py",
-        version="0.1.0",
-        api_prefix=api_prefix_v1,
-    )
-
-
-# Shutdown event
-@app.on_event("shutdown")
-async def shutdown_event():
-    logger.info("Application shutdown initiated")