fix(document-processors): inline/external parity when structured tier is absent

Address round-3 review on PR #914:
- When a glyph-corrupt doc's structured rung is NOT registered, the inline path
  now falls through to OCR (with reason corrupt_glyphs), mirroring the external
  next_available_tier instead of silently keeping the fast result. A structured
  parse FAILURE remains terminal (tracked via structured_failed), matching the
  external path which does not escalate a failure. Added a debug log for the
  unregistered case and "(OCR not attempted)" to the failure warning.
- Tests: inline + external glyph-corrupt fallthrough to OCR when structured is
  unregistered; glyph-corrupt + junk-quality both-flags precedence (structured
  wins over the bad_text_layer/ocr route).
- Note the total_chars>0 mutual-exclusion with the scanned branch in
  _route_from_signals.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/document_processors/classifier.py

@@ -203,6 +203,8 @@ def _route_from_signals(
     Flags are diagnostic and independent of the verdict (e.g. ``image_heavy``
     fires on ANY image-heavy page; the OCR route needs a page FRACTION).
     """
+    # total_chars > 0 also guarantees this never overlaps the scanned branch
+    # (total_chars == 0), so a doc is never both glyph-corrupt and "scanned".
     glyph_corrupt = total_chars > 0 and control_ratio > glyph_corruption_ratio
 
     flags: set[str] = set()