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/_imaging_tiles.html

@@ -51,8 +51,8 @@
 {% macro floor_row(s) %}
   {% set is_inferred = (s.source == 'inferred') %}
   {% set is_blancco = (s.source == 'blancco' or s.pctype == 'blancco') %}
-  {% set stage_idx = s.stage_index | int(0) %}
-  {% set stage_total = s.stage_total | int(0) %}
+  {% set stage_idx = (s.stage_index or 0) | int(0) %}
+  {% set stage_total = (s.stage_total or 0) | int(0) %}
   {% set state = s.floor_state or 'running' %}
   {% set drive_pct = None %}
   {% if is_blancco and stage_idx == 4 and s.current_stage and '%]' in s.current_stage %}
@@ -178,7 +178,8 @@
         {% if is_blancco %}<a href="{{ url_for('blancco_reports') }}">Erasure reports</a>{% endif %}
         <div class="sep"></div>
         <form method="post" action="{{ url_for('imaging_delete_session', serial=s.serial) }}"
-              onsubmit="return confirm('Clear the session for {{ s.serial }}? Its status and log files are deleted.');">
+              data-confirm="Clear the session for {{ s.serial }}? Its status and log files are deleted."
+              onsubmit="return confirm(this.dataset.confirm);">
           <input type="hidden" name="_csrf_token" value="{{ csrf_token() }}">
           <button type="submit" class="del">Clear session</button>
         </form>