Webapp redesign: fixes from the code review

Blocker
- /reports/export returned 500: REPORT_EXPORT_RANGES and _months_ago were
  deleted with the old reports route. Restored.

Major
- A session without stage_index/stage_total took down /, /imaging and
  /imaging/tiles (int filter on Undefined). The templates default both;
  saving a LAPS password for a session that no longer exists is refused
  (it created a bare session).
- /reports/todo parsed every report four times (23 s at 700 reports).
  blancco_report.parse() now caches by path, mtime and size: 0.09 s.
- Stored XSS through the session serial in the Clear confirm (the
  browser decodes ' before running an inline handler). The message
  moves to data-confirm, and update_session() only accepts serials of
  letters, digits, '.', '_' and '-' (400 from /imaging/status otherwise).
- Reports without a document id and with odd characters in the file name
  got a key the mark routes rejected, so they could never be cleared.
  report_key() now always produces a valid key.

Minor
- failure_note() handles a list-valued erasure target (500 on the report
  and to-do pages).
- _recently_marked() tolerates naive, 'Z' and non-dict state entries.
- Key regexes use fullmatch (a trailing newline was accepted).
- Unreadable reports are no longer counted as to tick / to destroy; the
  reports page already flags them as Unreadable.
- Rollback review recounts restorable files after reading the backups.
- Enrollment: the startnet fallback package (no or empty ppkg.conf) and a
  quoted SOURCE_PPKG are protected too.
- Backup upload overwrite check is case-insensitive (SMB share) and
  replaces the existing file rather than adding a case twin.
- Overview polling keeps the last good rows on an error and pauses while
  the tab is hidden. A shown or typed LAPS password survives the floor's
  refresh.
- hardware_dismiss only redirects to same-site paths; tiles ?limit is
  clamped; the Updates overview survives an unreadable Dell scan or iPXE
  state; describe() uses aversion(); a client cannot label its session
  'inferred'.

webapp/templates/dashboard.html

@@ -208,8 +208,11 @@
 (function () {
     var url = "{{ url_for('imaging_tiles_partial', limit=6) }}";
     setInterval(function () {
-        if (document.querySelector('#imaging-tiles details[open], #imaging-tiles details.more[open]')) return;
-        fetch(url, {credentials: 'same-origin'}).then(function (r) { return r.text(); }).then(function (html) {
+        if (document.hidden || document.querySelector('#imaging-tiles details[open], #imaging-tiles details.more[open]')) return;
+        fetch(url, {credentials: 'same-origin'}).then(function (r) {
+            if (!r.ok) throw new Error('HTTP ' + r.status);  // keep the last good rows
+            return r.text();
+        }).then(function (html) {
             var box = document.getElementById('imaging-tiles');
             if (box) { box.innerHTML = html; if (window.renderAllQRs) window.renderAllQRs(); }
         }).catch(function () {});