Make Stop stop: no replay, no restart, immediate feedback

Pressing Stop could look like the scan restarting, and for scheduled jobs
it really did restart the Google stage.

- Scheduler: check the abort flag between stages and never clear the
  Google abort after a Stop (that started a fresh all-users Google scan).
  A stopped job is recorded as "cancelled" and skips email/retention;
  scheduler_done carries cancelled=true.
- SSE: scan_cancelled and file_scan_done now end the current scan like
  scan_done, and sse.replay_messages() never replays a finished scan.
  Previously a stopped M365 scan (and every file scan) stayed "current",
  so each reconnect replayed it and re-showed Stop + progress.
- Frontend: scan_cancelled clears the M365 running flag and only tears
  down the UI when no Google/file scan is still running; scheduler_done /
  scheduler_error clear all running flags.
- run_scan: Stop during collection now ends the scan (checkpoint, finish,
  scan_cancelled) instead of announcing scan_start / "Scanning…".
- Stop button: switches to a disabled "Stopping…" state on click; all
  show/hide goes through _setStopBtnDisplay(), which resets it when hidden
  or when a new scan starts.

Adds tests/test_stop_scan.py (10 tests).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

static/js/results.js

@@ -779,7 +779,7 @@ function _sseWatchdog() {
       _ensureSSE();
       if (status.running && !S._m365ScanRunning && !S._googleScanRunning && !S._fileScanRunning) {
         document.getElementById('scanBtn').disabled = true;
-        document.getElementById('stopBtn').style.display = 'inline-block';
+        _setStopBtnDisplay('inline-block');
         // status.running reflects the M365 + file lock; treat as an M365 reconnect
         S._m365ScanRunning = true; _renderProgressSegments();
         document.getElementById('progressFile').textContent = t('m365_sse_reconnecting', 'Reconnecting to running scan…');