feat: garde-fous d'écriture des classeurs Excel #153 (P0)

L'édition d'un .xlsx pouvait détruire une partie du classeur, le
concurrencer en silence, ou diffuser une injection de formule.

- BUG-085 : inspect_workbook() détecte ce qu'un round-trip openpyxl perd
  (valeurs calculées en cache, slicers, contrôles, connexions, custom
  XML, signature, commentaires enrichis, macros) → xlsx_lossy_features
  exposé en lecture, bandeau FR/EN, et 409 xlsx_lossy_content sans
  `force` (confirmation explicite puis reprise). Périmètre réel
  revalidé : graphiques, images et TCD survivent au round-trip.
- BUG-086 : écriture atomique (fichier .tmp + os.replace) : un plantage
  ne peut plus tronquer le classeur, le backup reste intact.
- BUG-087 : verrou par fichier autour du read-modify-write (timeout 15 s,
  409 conflict) ; endpoint xlsx/save devenu synchrone pour que
  l'attente s'exécute dans le threadpool.
- BUG-088 : une saisie en '=' ou '@' est stockée en texte, sauf opt-in
  `allow_formula` ou le bouton f(x) de la visionneuse. Le handler
  ServiceError expose désormais code + details, que api() propage.
- BUG-084 : la suppression d'une vault purge enfin l'index inversé
  (documents fantômes qui continuaient de matcher) et is_stale() devient
  is_ready(), le nom étant trompeur (la staleness n'existe plus).

Tests : 1390 pytest, 10 JSDOM (xlsx-viewer.test.mjs, branché au CI),
3 E2E Playwright, suite E2E complète verte, ruff/mypy 0.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <[email protected]>

backend/xlsx_reader.py

@@ -3,11 +3,16 @@
 Read-only: formulas are shown as their text (``data_only=False``) so a
 round-trip through the viewer never depends on Excel's cached values.
 Write-side lives in ``backend.services.mutations.edit_xlsx_cells``.
+
+:func:`inspect_workbook` lists the workbook features that an openpyxl
+round-trip would drop (#153 A1) so the UI can warn before saving.
 """
 
 from __future__ import annotations
 
 import html
+import re
+import zipfile
 from datetime import date, datetime
 from pathlib import Path
 from typing import Any
@@ -20,6 +25,29 @@ from openpyxl.utils import get_column_letter
 MAX_ROWS = 500
 MAX_COLS = 40
 
+# #153 A1 — workbook parts openpyxl does not re-serialize on load+save.
+# Verified against openpyxl 3.1.5: charts, images, drawings and pivot tables
+# DO survive the round-trip, so they are deliberately absent from this map.
+LOSSY_PARTS: dict[str, tuple[str, ...]] = {
+    "slicers": ("xl/slicers/", "xl/slicerCaches/", "xl/timelines/"),
+    "form_controls": ("xl/ctrlProps/", "xl/activeX/"),
+    "connections": ("xl/queryTables/", "xl/connections.xml"),
+    "custom_xml": ("customXml/",),
+    "signature": ("_xmlsignatures/",),
+    "rich_comments": ("xl/threadedComments/", "xl/persons/"),
+    "macros": ("xl/vbaProject.bin",),
+}
+
+# A formula cell carrying its last computed result: ``<f>…</f><v>…</v>``.
+# openpyxl writes an EMPTY ``<v></v>`` itself, hence the ``[^<]`` guard: only a
+# non-empty value counts. openpyxl keeps the formula but drops the cached result,
+# so any reader using ``data_only=True`` (pandas, converters) sees ``None`` until
+# Excel recalculates.
+_CACHED_FORMULA_RE = re.compile(rb"<f[ >][^<]*</f>\s*<v>[^<]")
+
+# Sheet XML scanned by the cached-formula probe (CPU guard, like MAX_REPLACE_FILE_BYTES).
+_MAX_PROBE_BYTES = 8_000_000
+
 
 def _fmt(value: Any) -> str:
     if value is None:
@@ -68,6 +96,51 @@ def _table(grid: list[list[str]]) -> str:
     return "".join(out)
 
 
+def _has_cached_formulas(zf: zipfile.ZipFile) -> bool:
+    """True when at least one formula cell still carries its computed value."""
+    budget = _MAX_PROBE_BYTES
+    for name in zf.namelist():
+        if not name.startswith("xl/worksheets/sheet") or not name.endswith(".xml"):
+            continue
+        try:
+            with zf.open(name) as fh:
+                while budget > 0:
+                    chunk = fh.read(65536)
+                    if not chunk:
+                        break
+                    budget -= len(chunk)
+                    if _CACHED_FORMULA_RE.search(chunk):
+                        return True
+        except (KeyError, OSError, zipfile.BadZipFile):
+            continue
+    return False
+
+
+def inspect_workbook(file_path: Path) -> list[str]:
+    """Return the sorted keys of :data:`LOSSY_PARTS` present in *file_path*.
+
+    Read-only inspection of the OPC package (central directory + a bounded scan
+    of the sheet XML). Never raises: an unreadable or encrypted workbook simply
+    yields ``[]`` and the save path keeps its current behaviour.
+
+    ``cached_values`` is a synthetic key: openpyxl keeps the formula but drops
+    the cached result, so the workbook stays correct once Excel recalculates it.
+    """
+    try:
+        with zipfile.ZipFile(file_path) as zf:
+            names = set(zf.namelist())
+            found = {
+                key
+                for key, prefixes in LOSSY_PARTS.items()
+                if any(name.startswith(prefix) for name in names for prefix in prefixes)
+            }
+            if _has_cached_formulas(zf):
+                found.add("cached_values")
+            return sorted(found)
+    except (OSError, zipfile.BadZipFile):
+        return []
+
+
 def render_sheets(file_path: Path) -> list[dict[str, str]]:
     """Return ``[{"name": sheet_title, "html": table_html}, ...]``."""
     wb = load_workbook(str(file_path), read_only=True, data_only=False)