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/services/blancco_report.py

@@ -18,6 +18,7 @@ Returned shape (best-effort; missing keys are OK in templates):
     }
 """
 from __future__ import annotations
+import os
 import xml.etree.ElementTree as ET
 
 
@@ -52,7 +53,28 @@ def _text(parent, path, default=""):
     return el.text.strip()
 
 
+# Parsed reports by path, reused while the file's size and mtime are
+# unchanged. The reports list, the to-do (which walks a year of reports
+# several times per request), exports and evidence packs all parse the same
+# files; a report on the share is never edited in place. Callers must not
+# mutate the returned dict.
+_parse_cache: dict = {}
+
+
 def parse(path: str) -> dict:
+    st = os.stat(path)
+    sig = (st.st_mtime_ns, st.st_size)
+    hit = _parse_cache.get(path)
+    if hit and hit[0] == sig:
+        return hit[1]
+    data = _parse(path)
+    if len(_parse_cache) > 5000:  # far above a site's report count; just a bound
+        _parse_cache.clear()
+    _parse_cache[path] = (sig, data)
+    return data
+
+
+def _parse(path: str) -> dict:
     tree = ET.parse(path)
     root = tree.getroot()
     rep = root.find(".//report/blancco_data") or root