Add task snapshots, inspection, and diffing to aiomonitor

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What this task asks for

Add point-in-time task snapshots with interactive inspection, diffing, deletion, and web/CLI access. Add snapshots to Monitor freezing running and terminated task state. IDs auto-increment from 1 with optional name. Monitor/start_monitor accept max_snapshots (default 10), evicting oldest unnamed first, preserving named. Add snapshot CLI group using the existing command dispatch loop and completion signaling, with error feedback on invalid IDs: save(--name, echoed in output), list(ls), show, where, diff, delete, plus web endpoints and /snapshots nav page. Web API JSON at /api/snapshot/: save(POST, returns {id}), list(GET, returns {snapshots}), tasks(POST snapshot_id, returns {tasks}), trace(POST snapshot_id + task_id), diff(POST snapshot_id_1 + snapshot_id_2, returns {added, removed, common}). Delete: DELETE /api/snapshot (query snapshot_id), 404/400 when missing.