- 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
