feat(inbox): docx table cells + grouped pptx shapes (stage 2g)

Two structural regions the office extractors missed, no new dependency:
- .docx table cells (they live outside doc.paragraphs);
- grouped .pptx shapes (add_group_shape moves a shape inside the group, so the
  extractor recurses through MSO_SHAPE_TYPE.GROUP to reach it).

Each proven by an injection-in-region REJECT plus a clean-region ADMIT. Tests
310 -> 314.

tests/inbox_frontend.py

@@ -137,6 +137,13 @@ def _extract_docx_text(fs_path) -> str:
     doc = Document(str(fs_path))
     parts = [para.text for para in doc.paragraphs if para.text]
 
+    # Table cells live outside doc.paragraphs — iterate them explicitly.
+    for table in doc.tables:
+        for row in table.rows:
+            for cell in row.cells:
+                if cell.text:
+                    parts.append(cell.text)
+
     cp = doc.core_properties
     for attr in ("title", "subject", "keywords", "comments", "category", "author"):
         value = getattr(cp, attr, None)
@@ -165,11 +172,21 @@ def _extract_pptx_text(fs_path) -> str:
     image/shape alt-text. ``python-pptx`` is imported lazily (dev/showcase-scoped).
     """
     from pptx import Presentation
+    from pptx.enum.shapes import MSO_SHAPE_TYPE
+
+    def walk(shapes):
+        # Flatten grouped shapes: add_group_shape moves a shape inside the group,
+        # so only recursion reaches its text/alt-text.
+        for shape in shapes:
+            if shape.shape_type == MSO_SHAPE_TYPE.GROUP:
+                yield from walk(shape.shapes)
+            else:
+                yield shape
 
     prs = Presentation(str(fs_path))
     parts: list = []
     for slide in prs.slides:
-        for shape in slide.shapes:
+        for shape in walk(slide.shapes):
             if shape.has_text_frame and shape.text_frame.text:
                 parts.append(shape.text_frame.text)   # incl. off-slide boxes
             alt = _shape_alt_text(shape)