feat(FN-1153): add cross-tab locks for AI planning sessions

- Bump SQLite schema to v19 with ai_sessions lock columns and lock index, and align migration coverage in core DB tests
- Extend AiSessionStore with acquire/release/force lock APIs, stale lock cleanup, and lock metadata in ai_session update summaries
- Enforce lock checks on planning, subtask, and mission interview mutation routes with 409 conflict responses while keeping stream reads unaffected
- Add frontend tab identity + useSessionLock hook and wire Planning, Subtask, and Mission modals to pass tabId, show lock overlay, and support Take Control
- Expand dashboard route/e2e and modal tests to validate lock enforcement, lock handoff, and lock-aware session reentry behavior

packages/dashboard/app/components/SubtaskBreakdownModal.test.tsx

@@ -7,6 +7,9 @@ const mockRetrySubtaskSession = vi.fn();
 const mockConnectSubtaskStream = vi.fn();
 const mockCreateTasksFromBreakdown = vi.fn();
 const mockCancelSubtaskBreakdown = vi.fn();
+const mockAcquireSessionLock = vi.fn();
+const mockReleaseSessionLock = vi.fn();
+const mockForceAcquireSessionLock = vi.fn();
 
 vi.mock("../api", () => ({
   startSubtaskBreakdown: (...args: any[]) => mockStartSubtaskBreakdown(...args),
@@ -14,6 +17,9 @@ vi.mock("../api", () => ({
   connectSubtaskStream: (...args: any[]) => mockConnectSubtaskStream(...args),
   createTasksFromBreakdown: (...args: any[]) => mockCreateTasksFromBreakdown(...args),
   cancelSubtaskBreakdown: (...args: any[]) => mockCancelSubtaskBreakdown(...args),
+  acquireSessionLock: (...args: any[]) => mockAcquireSessionLock(...args),
+  releaseSessionLock: (...args: any[]) => mockReleaseSessionLock(...args),
+  forceAcquireSessionLock: (...args: any[]) => mockForceAcquireSessionLock(...args),
 }));
 
 vi.mock("../hooks/modalPersistence", () => ({
@@ -49,6 +55,9 @@ describe("SubtaskBreakdownModal", () => {
     });
     mockCreateTasksFromBreakdown.mockResolvedValue({ tasks: [{ id: "FN-101" }, { id: "FN-102" }] });
     mockCancelSubtaskBreakdown.mockResolvedValue(undefined);
+    mockAcquireSessionLock.mockResolvedValue({ acquired: true, currentHolder: null });
+    mockReleaseSessionLock.mockResolvedValue(undefined);
+    mockForceAcquireSessionLock.mockResolvedValue({ acquired: true, currentHolder: null });
     vi.stubGlobal("confirm", vi.fn(() => true));
   });
 
@@ -73,6 +82,27 @@ describe("SubtaskBreakdownModal", () => {
     expect(await screen.findByText("AI is generating subtasks...")).toBeInTheDocument();
   });
 
+  it("shows lock overlay and allows take-control", async () => {
+    window.sessionStorage.setItem("fusion-tab-id", "tab-self");
+    mockAcquireSessionLock.mockResolvedValueOnce({ acquired: false, currentHolder: "tab-other" });
+
+    renderModal();
+
+    await waitFor(() => {
+      expect(screen.getByTestId("session-lock-overlay")).toBeInTheDocument();
+    });
+
+    fireEvent.click(screen.getByText("Take Control"));
+
+    await waitFor(() => {
+      expect(mockForceAcquireSessionLock).toHaveBeenCalledWith("session-123", "tab-self");
+    });
+
+    await waitFor(() => {
+      expect(screen.queryByTestId("session-lock-overlay")).not.toBeInTheDocument();
+    });
+  });
+
   it("hides send to background button in initial state", () => {
     render(
       <SubtaskBreakdownModal
@@ -495,7 +525,7 @@ describe("SubtaskBreakdownModal", () => {
       fireEvent.click(retryButton);
 
       await waitFor(() => {
-        expect(mockRetrySubtaskSession).toHaveBeenCalledWith("session-123", undefined);
+        expect(mockRetrySubtaskSession).toHaveBeenCalledWith("session-123", undefined, expect.any(String));
       });
       expect(mockConnectSubtaskStream).toHaveBeenCalledTimes(2);
     });