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/enrollment.html

@@ -109,7 +109,7 @@
   {% if packages %}
   <div class="sgrid colhead" aria-hidden="true"><span>File</span><span class="num">Size</span><span class="when">Modified</span></div>
   {% for p in packages %}
-  {% set is_active = active_ppkg and active_ppkg.SOURCE_PPKG and p.filename|lower == active_ppkg.SOURCE_PPKG|lower %}
+  {% set is_active = p.filename|lower == effective_ppkg|lower %}
   <div class="ui-row">
     <div class="sgrid">
       <span class="s-name">