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/README.md

@@ -26,7 +26,6 @@ Outputs:
 | --- | --- |
 | `unparsed-dates.csv` | For each `raw` (sorted by frequency), fill `suggested_iso` + `suggested_precision`, then paste `raw,suggested_iso,suggested_precision` into `overrides/dates.csv` (header `raw,iso,precision`). |
 | `unresolved-names.csv` | Names whose value is itself problematic, grouped by `category`: `unknown` (`?`/illegible), `single_token` (first OR last name only), `relational` (`Tante …`), `collective` (`Familie …`), `prose` (a description landed in a name column), `ambiguous_pair` (two given names → likely two people, not auto-split). Review highest-impact categories first; add decisions to `overrides/names.csv` (look up valid ids in `out/canonical-persons.xlsx`). |
-| `index-file-mismatch.csv` | The `Datei` path disagrees with the index-derived filename — reconcile when the PDFs arrive. |
 | `duplicate-index.csv`, `blank-index-rows.csv`, `skipped-x-suffix.csv` | Inspect; fix in the source spreadsheet if needed. |
 
 > `unresolved-names.csv` is the focused "names that need a human" list. Non-family