feat(FN-1148): add AI session keep-alive pings for SSE streams

- Add AiSessionStore.ping() and expose POST /api/ai-sessions/:id/ping for lightweight heartbeat updates without emitting high-frequency session events
- Add pingSession() client API and integrate 25s keep-alive timers into planning, subtask, and mission interview resilient SSE connections
- Ensure keep-alive timers stop on stream completion, fatal errors, and explicit close while treating ping failures as best-effort non-fatal behavior
- Expand API/store/routes coverage with targeted tests for ping endpoint behavior, ping store semantics, and SSE keep-alive lifecycle handling
- Stabilize MissionStore latest-error lookup ordering with a rowid tiebreaker when timestamps are equal

packages/dashboard/src/ai-session-store.test.ts

@@ -205,6 +205,30 @@ describe("AiSessionStore", () => {
     expect(projectA.every((session) => session.projectId === "project-a")).toBe(true);
   });
 
+  it("ping updates updatedAt for existing sessions without emitting updates", () => {
+    seedSession({ id: "S-ping", status: "awaiting_input" });
+
+    const staleTs = new Date(Date.now() - 60_000).toISOString();
+    db.prepare("UPDATE ai_sessions SET updatedAt = ? WHERE id = ?").run(staleTs, "S-ping");
+
+    const onUpdated = vi.fn();
+    store.on("ai_session:updated", onUpdated);
+
+    const updated = store.ping("S-ping");
+
+    expect(updated).toBe(true);
+    expect(store.get("S-ping")?.updatedAt).not.toBe(staleTs);
+    expect(onUpdated).not.toHaveBeenCalled();
+  });
+
+  it("ping returns false for nonexistent sessions", () => {
+    const onUpdated = vi.fn();
+    store.on("ai_session:updated", onUpdated);
+
+    expect(store.ping("missing-session")).toBe(false);
+    expect(onUpdated).not.toHaveBeenCalled();
+  });
+
   it("listRecoverable returns awaiting_input and generating sessions", () => {
     seedSession({ id: "S-generating", status: "generating", ageMs: 3_000 });
     seedSession({ id: "S-awaiting", status: "awaiting_input", ageMs: 1_000 });

packages/dashboard/src/ai-session-store.ts

@@ -145,6 +145,20 @@ export class AiSessionStore extends EventEmitter<AiSessionStoreEvents> {
     return row ?? null;
   }
 
+  /**
+   * Lightweight heartbeat for active sessions.
+   * Updates only `updatedAt` and intentionally does NOT emit
+   * `ai_session:updated` to avoid high-frequency SSE broadcasts.
+   */
+  ping(id: string): boolean {
+    const now = new Date().toISOString();
+    const result = this.db
+      .prepare("UPDATE ai_sessions SET updatedAt = ? WHERE id = ?")
+      .run(now, id) as { changes?: number };
+
+    return Number(result.changes ?? 0) > 0;
+  }
+
   /**
    * List active sessions (generating or awaiting_input).
    * Optionally filtered by projectId.

packages/dashboard/src/routes.test.ts

@@ -6366,6 +6366,46 @@ describe("Git Management endpoints", () => {
   });
 });
 
+describe("POST /api/ai-sessions/:id/ping", () => {
+  let store: TaskStore;
+
+  beforeEach(() => {
+    store = createMockStore();
+  });
+
+  it("returns 200 when the session exists", async () => {
+    const mockAiSessionStore = {
+      ping: vi.fn().mockReturnValue(true),
+    };
+
+    const app = express();
+    app.use(express.json());
+    app.use("/api", createApiRoutes(store, { aiSessionStore: mockAiSessionStore as any }));
+
+    const res = await REQUEST(app, "POST", "/api/ai-sessions/session-123/ping");
+
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual({ ok: true });
+    expect(mockAiSessionStore.ping).toHaveBeenCalledWith("session-123");
+  });
+
+  it("returns 404 when the session does not exist", async () => {
+    const mockAiSessionStore = {
+      ping: vi.fn().mockReturnValue(false),
+    };
+
+    const app = express();
+    app.use(express.json());
+    app.use("/api", createApiRoutes(store, { aiSessionStore: mockAiSessionStore as any }));
+
+    const res = await REQUEST(app, "POST", "/api/ai-sessions/missing-session/ping");
+
+    expect(res.status).toBe(404);
+    expect(res.body).toEqual({ error: "Session not found" });
+    expect(mockAiSessionStore.ping).toHaveBeenCalledWith("missing-session");
+  });
+});
+
 describe("Terminal session routes", () => {
   let store: TaskStore;
 

packages/dashboard/src/routes.ts

@@ -8469,6 +8469,26 @@ Output ONLY the prompt text (no markdown, no explanations).`;
     res.json(session);
   });
 
+  /**
+   * POST /api/ai-sessions/:id/ping
+   * Lightweight keep-alive touch for active AI sessions.
+   */
+  router.post("/ai-sessions/:id/ping", (req, res) => {
+    if (!aiSessionStore) {
+      res.status(404).json({ error: "AI sessions not available" });
+      return;
+    }
+
+    const { id } = req.params;
+    const updated = aiSessionStore.ping(id);
+    if (!updated) {
+      res.status(404).json({ error: "Session not found" });
+      return;
+    }
+
+    res.json({ ok: true });
+  });
+
   /**
    * DELETE /api/ai-sessions/:id
    * Dismiss/cancel a background AI session.