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/src/subtask-breakdown.ts

@@ -196,6 +196,8 @@ function persistSubtaskSession(session: SubtaskInternalSession, status: "generat
     projectId: null,
     createdAt: session.createdAt.toISOString(),
     updatedAt: new Date().toISOString(),
+    lockedByTab: null,
+    lockedAt: null,
   };
   _aiSessionStore.upsert(row);
 }