Add scheduler for recurring and single-window audit runs

[SwiftWing21]

The audit engine needs a scheduler that supports both recurring intervals (1-30 day cadence) and single time-window blocks. Implement scheduler in supervisor with fleet.toml config for schedule, timezone, and blackout windows.

[SwiftWing21]

Partial: supervisor.py \ polling is fully implemented (lines 83-132) and /\ in fleet/manual_mode.py (lines 473-496) provide the backend. However, mod_manual_mode.py has no Scheduler tab — the tabview only has 'Queue Builder', 'Run Controls', and 'Results'. Scheduler configuration requires direct fleet.toml editing. UI tab for recurring/one-time schedule setup still missing.