refactor(normalizer): drop file column now PDFs resolve by index

The import corpus is uniform: every PDF is named <index>.pdf, so the
file column (the spreadsheet's datei value) is redundant. Remove file
from CanonicalDocument, RawRow, _FIELDS, to_canonical, and DOC_COLUMNS,
plus the now-moot index_file_mismatch review flag/CSV/stat and the
datei header mapping. date_end and the tree person_id are kept.

Refs #686

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

tools/import-normalizer/config.py

@@ -18,7 +18,6 @@ OVERRIDES_DIR = BASE_DIR / "overrides"
 # --- Header text (lowercased, whitespace-collapsed) -> canonical field ---
 DOCUMENT_HEADER_MAP = {
     "index": "index",
-    "datei": "file",
     "box": "box",
     "mappe": "folder",
     "briefeschreiberin": "sender",