fix: harden pi maintenance lifecycle

backend/src/routes/sessions.ts

@@ -37,6 +37,8 @@ export function sessionRoutes(
     legacyWorkspaceMode?: boolean;
     /** Fail-closed installation/runtime transport capability check. */
     workspaceRuntimeSupport: (workspace: WorkspaceDescriptor) => boolean;
+    /** Host-controlled admission guard. Existing runtimes deliberately continue. */
+    maintenanceGate: () => boolean;
   },
 ) {
   const lifecycleTails = new Map<string, Promise<void>>();
@@ -75,6 +77,11 @@ export function sessionRoutes(
     return typeof runner.withPrincipal === "function" ? runner.withPrincipal(principal) : runner;
   };
 
+  const maintenanceReply = (reply: any) => reply.code(503).send({
+    code: "maintenance",
+    error: "Session admission is temporarily paused for maintenance. Try again shortly.",
+  });
+
   /** Include retained historical descriptors so removed workspaces remain resumable. */
   const sessionRevisions = async () => {
     const registry = d.workspaceRegistry as Partial<WorkspaceRegistry>;
@@ -272,6 +279,7 @@ export function sessionRoutes(
   });
 
   app.post("/sessions", async (req, reply) => {
+    if (d.maintenanceGate()) return maintenanceReply(reply);
     const b = req.body as {
       question: string; name?: string; workspace?: string; workspaceId?: string;
       provider?: string; model?: string; thinking?: string;
@@ -502,6 +510,7 @@ export function sessionRoutes(
     return reply.code(204).send();
   });
   app.post("/sessions/:id/resume", async (req, reply) => {
+    if (d.maintenanceGate()) return maintenanceReply(reply);
     const id = (req.params as any).id;
     const principal = getPrincipal(req);
     return withSessionLifecycle(id, async () => {