feat(backend): add pricing coverage, unpriced seats, and explain endpoints

add backend endpoints for pricing coverage analysis and unpriced seat inspection

add explain endpoint to make effective pricing decisions traceable
improve pricing diagnostics for admin and editor workflows

backend/app/api/routes/__init__.py

@@ -4,6 +4,7 @@ from app.api.routes.admin import router as admin_router
 from app.api.routes.audit import router as audit_router
 from app.api.routes.editor import router as editor_router
 from app.api.routes.pricing import router as pricing_router
+from app.api.routes.pricing_diagnostics import router as pricing_diagnostics_router
 from app.api.routes.publish import router as publish_router
 from app.api.routes.schemes import router as schemes_router
 from app.api.routes.structure import router as structure_router
@@ -17,6 +18,7 @@ router.include_router(uploads_router)
 router.include_router(schemes_router)
 router.include_router(structure_router)
 router.include_router(pricing_router)
+router.include_router(pricing_diagnostics_router)
 router.include_router(test_mode_router)
 router.include_router(audit_router)
 router.include_router(admin_router)