feat(scanner): fill sparse sidecars from the PDF Info dictionary

Same gap-fill rule as the EPUB path, against the PDF's embedded Info
dictionary via a readPDFInfoDict helper reusing extractPDFMetadata's
field conventions (creator falls back to author, subject maps to
description, producer to publisher, keywords to tags, plus page count).
Sidecar values always win; unopenable PDFs skip silently.

TestMergeMetadataPDFGapFill uses an Info-bearing hand-built PDF fixture
(shared with the sidecar-cover test) and asserts the sidecar title is
kept while author/description/publisher/tags/page count fill in.

internal/services/media_scanner_metadata_test.go

@@ -262,13 +262,18 @@ func createTestPDFWithImage() []byte {
 	writeObj(5, func(w *bytes.Buffer) {
 		fmt.Fprintf(w, "<< /Length %d >>\nstream\n%s\nendstream\n", len(content), content)
 	})
+	writeObj(6, func(w *bytes.Buffer) {
+		// Info dictionary deliberately richer than the sidecars in gap-fill
+		// tests: gap-fill must use these, never overwrite with them.
+		w.WriteString("<< /Title (Embedded Title) /Author (Embedded Author) /Subject (Embedded Subject) /Producer (Embedded Producer) /Keywords (embedded-kw) >>\n")
+	})
 
 	xrefStart := buf.Len()
 	fmt.Fprintf(&buf, "xref\n0 %d\n0000000000 65535 f \n", len(offsets))
 	for _, off := range offsets[1:] {
 		fmt.Fprintf(&buf, "%010d 00000 n \n", off)
 	}
-	fmt.Fprintf(&buf, "trailer\n<< /Size %d /Root 1 0 R >>\nstartxref\n%d\n%%%%EOF\n", len(offsets), xrefStart)
+	fmt.Fprintf(&buf, "trailer\n<< /Size %d /Root 1 0 R /Info 6 0 R >>\nstartxref\n%d\n%%%%EOF\n", len(offsets), xrefStart)
 	return buf.Bytes()
 }
 
@@ -363,3 +368,43 @@ func TestMergeMetadataEPUBGapFill(t *testing.T) {
 		t.Errorf("Language = %q, want embedded fill", merged.Language)
 	}
 }
+
+// TestMergeMetadataPDFGapFill verifies the same for PDFs: a sparse sidecar
+// keeps its title while author/description/publisher/tags/pagecount fill in
+// from the embedded Info dictionary.
+func TestMergeMetadataPDFGapFill(t *testing.T) {
+	dir := t.TempDir()
+	pdfPath := filepath.Join(dir, "gappy.pdf")
+	if err := os.WriteFile(pdfPath, createTestPDFWithImage(), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	sparse := &MediaMetadata{Title: "Kept Title"}
+	s := NewMediaScanner(nil)
+	merged, err := s.mergeMetadata(pdfPath, sparse)
+	if err != nil {
+		t.Fatalf("mergeMetadata() error: %v", err)
+	}
+
+	checks := []struct {
+		name string
+		got  string
+		want string
+	}{
+		{"Title (sidecar wins)", merged.Title, "Kept Title"},
+		{"Author", merged.Author, "Embedded Author"},
+		{"Description", merged.Description, "Embedded Subject"},
+		{"Publisher", merged.Publisher, "Embedded Producer"},
+	}
+	for _, c := range checks {
+		if c.got != c.want {
+			t.Errorf("%s = %q, want %q", c.name, c.got, c.want)
+		}
+	}
+	if len(merged.Tags) == 0 {
+		t.Error("Tags empty, want keywords from Info dict")
+	}
+	if merged.PageCount != 1 {
+		t.Errorf("PageCount = %d, want 1 from Info dict", merged.PageCount)
+	}
+}