fix(scanner): classify .docx as reflowable — was landing format_group unknown

.docx is accepted for ingest but the format-group switch had no case
for it, so every DOCX row stored format_group 'unknown' with
is_reflowable=false. Extract the classification switch into a pure
classifyFormatGroup helper with a table test, and classify .docx
reflowable like the other reflowable text formats (.rtf/.doc stay out
until they are readable client-side). Existing rows: dev alice (DOCX)
reclassified by hand; production rows reclassify on rescan.

internal/services/media_scanner_test.go

@@ -119,3 +119,34 @@ func TestProcessDirtyDirectories_CollectsReadyDirs(t *testing.T) {
 	scanner.dirtyDirsMu.RUnlock()
 	assert.Equal(t, 0, count, "Ready directory should be removed from dirty list")
 }
+
+func TestClassifyFormatGroup(t *testing.T) {
+	tests := []struct {
+		name      string
+		ext       string
+		epubFixed bool
+		group     string
+		reflow    bool
+		fixed     bool
+	}{
+		{"EPUB reflowable", ".epub", false, "reflowable", true, false},
+		{"EPUB fixed-layout", ".epub", true, "fixed_layout", false, true},
+		{"MOBI", ".mobi", false, "reflowable", true, false},
+		{"FB2", ".fb2", false, "reflowable", true, false},
+		{"TXT", ".txt", false, "reflowable", true, false},
+		{"DOCX (was unknown before the fix)", ".docx", false, "reflowable", true, false},
+		{"PDF", ".pdf", false, "fixed_layout", false, true},
+		{"DJVU", ".djvu", false, "fixed_layout", false, true},
+		{"CBZ", ".cbz", false, "comic_archive", false, true},
+		{"unrecognized extension", ".xyz", false, "unknown", false, false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			group, reflow, fixed := classifyFormatGroup(tt.ext, tt.epubFixed)
+			if group != tt.group || reflow != tt.reflow || fixed != tt.fixed {
+				t.Errorf("classifyFormatGroup(%q, %v) = (%q, %v, %v), want (%q, %v, %v)",
+					tt.ext, tt.epubFixed, group, reflow, fixed, tt.group, tt.reflow, tt.fixed)
+			}
+		})
+	}
+}