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/core/src/db.ts

@@ -59,7 +59,7 @@ export function fromJson<T>(json: string | null | undefined): T | undefined {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 18;
+const SCHEMA_VERSION = 19;
 
 function normalizeTaskComments(
   steeringComments: SteeringComment[] | undefined,
@@ -731,6 +731,17 @@ export class Database {
         this.db.exec(`CREATE INDEX IF NOT EXISTS idxTaskDocumentRevisionsTaskKey ON task_document_revisions(taskId, key)`);
       });
     }
+
+    if (version < 19) {
+      this.applyMigration(19, () => {
+        if (!this.hasTable("ai_sessions")) {
+          return;
+        }
+        this.addColumnIfMissing("ai_sessions", "lockedByTab", "TEXT");
+        this.addColumnIfMissing("ai_sessions", "lockedAt", "TEXT");
+        this.db.exec("CREATE INDEX IF NOT EXISTS idxAiSessionsLock ON ai_sessions(lockedByTab)");
+      });
+    }
   }
 
   /**