fix: serve FastAPI docs under /api prefix

Set docs_url=/api/docs, redoc_url=/api/redoc,
openapi_url=/api/openapi.json so Swagger UI is
accessible through nginx at http://localhost/api/docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -3,7 +3,13 @@ from fastapi.middleware.cors import CORSMiddleware
 
 from app.api import tests
 
-app = FastAPI(title="QA Test App API", version="0.1.0")
+app = FastAPI(
+    title="QA Test App API",
+    version="0.1.0",
+    docs_url="/api/docs",
+    redoc_url="/api/redoc",
+    openapi_url="/api/openapi.json",
+)
 
 app.add_middleware(
     CORSMiddleware,