Fix CI failures: add backend/conftest.py for module resolution and run black formatting

- Add backend/conftest.py that inserts the backend directory into sys.path,
  fixing ModuleNotFoundError when pytest runs from the backend/ directory
  (as CI does with `cd backend && pytest tests/`)
- Run black formatter on all 28 backend files that needed reformatting
- All 53 tests pass with both `pytest tests/` and `python -m pytest tests/`

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/beb47db2-da25-416a-8fb0-c1452a3b22a7

backend/app/main.py

@@ -1,6 +1,7 @@
 """
 Main FastAPI application.
 """
+
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.middleware.trustedhost import TrustedHostMiddleware
@@ -14,7 +15,7 @@ from app.api.v1.api import api_router
 # Configure logging
 logging.basicConfig(
     level=getattr(logging, settings.LOG_LEVEL.upper()),
-    format='%(asctime)s - %(name)s - %(levelname)s - %(message)s'
+    format="%(asctime)s - %(name)s - %(levelname)s - %(message)s",
 )
 
 logger = logging.getLogger(__name__)
@@ -22,20 +23,20 @@ logger = logging.getLogger(__name__)
 
 def create_application() -> FastAPI:
     """Create and configure FastAPI application"""
-    
+
     app = FastAPI(
         title=settings.APP_NAME,
         version=settings.APP_VERSION,
         description="Multi-tenant POP3/IMAP to Gmail forwarder with subscription management",
         docs_url="/api/docs",
         redoc_url="/api/redoc",
-        openapi_url="/api/openapi.json"
+        openapi_url="/api/openapi.json",
     )
-    
+
     # Security middleware (add before CORS)
     app.add_middleware(SecurityHeadersMiddleware)
     app.add_middleware(CSRFProtectionMiddleware)
-    
+
     # CORS middleware
     app.add_middleware(
         CORSMiddleware,
@@ -44,36 +45,36 @@ def create_application() -> FastAPI:
         allow_methods=["*"],
         allow_headers=["*"],
     )
-    
+
     # Include API router
     app.include_router(api_router, prefix=settings.API_V1_PREFIX)
-    
+
     @app.get("/")
     async def root():
         """Root endpoint"""
         return {
             "message": "POP3 Forwarder SaaS API",
             "version": settings.APP_VERSION,
-            "docs": "/api/docs"
+            "docs": "/api/docs",
         }
-    
+
     @app.get("/health")
     async def health_check():
         """Health check endpoint for container orchestration"""
         return {"status": "healthy"}
-    
+
     @app.on_event("startup")
     async def startup_event():
         """Run on application startup"""
         logger.info(f"Starting {settings.APP_NAME} v{settings.APP_VERSION}")
         logger.info(f"Debug mode: {settings.DEBUG}")
         logger.info(f"API documentation: /api/docs")
-    
+
     @app.on_event("shutdown")
     async def shutdown_event():
         """Run on application shutdown"""
         logger.info("Shutting down application")
-    
+
     return app