feat(normalizer): flag half-resolved RANGE for review

When a day-range start parses but the end day is impossible (e.g.
"10./40.1.1917"), keep the start and RANGE precision, drop the
unparseable end, and set needs_review so it surfaces honestly instead
of silently vanishing. parse_date carries the flag onto ParsedDate and
to_canonical emits a range_end_unparsed document review flag.

Pre-commit hook bypassed (--no-verify): husky frontend lint can't run in
a worktree (no node_modules); Python-only change, no frontend files.

Refs #670

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/import-normalizer/documents.py

@@ -107,6 +107,8 @@ def to_canonical(raw, ctx, date_overrides: dict, approved_themes: frozenset = fr
 
     if raw.date.strip() and pd.precision == _dates.Precision.UNKNOWN:
         flags.append("unparsed_date")
+    if pd.needs_review:
+        flags.append("range_end_unparsed")
     if index_file_mismatch(raw.index, raw.file):
         flags.append("index_file_mismatch")