v0.8.0-beta: Watch Mode / Scheduled Rechecks

[DeepZone]

Motivation

• Add a read-only Watch Mode to persist selected checks as Watch Targets and record repeated run history with change detection.
• Provide a robust manual / externally-triggerable recheck mechanism (a run-due endpoint) for scheduled rechecks without introducing an internal scheduler.

Description

• Added new SQLAlchemy models WatchTarget and WatchRun and an Alembic migration 0004_watch_mode to create watch_targets and watch_runs tables with FK links to users, change_cases and reports.
• Implemented backend API endpoints under /api/watch-targets for list/create/read/update/delete, runs listing, single-run (POST /api/watch-targets/{id}/run) and due-run execution (POST /api/watch-targets/run-due) with role enforcement (admin/operator mutate/run, viewer read-only).
• Implemented WatchService which reuses existing check services (BgpVisibilityService, RoaPlannerService, PrefixChecker, AsnChecker) to run checks, persist standard Check/Report, create WatchRun entries, set previous_status, compute changed, and update last_run_at / next_run_at / last_status.
• Added frontend API bindings, a new WatchModeView component, types, navigation wiring, and app version/docs bumps to v0.8.0-beta.

Testing

• Ran backend test suite with cd backend && pytest -q, including new backend/tests/test_watch_mode.py, and the tests completed successfully.
• Built the frontend with cd frontend && npm run build and the production build completed successfully.

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Codex Task