Fix ISBN normalization test expectations

Test expectations in TestMediaItemISBNNormalization were incorrect:
- Tests were expecting 12-digit outputs for 13-digit inputs
- Updated to expect correct 13-digit normalized outputs

This fixes the failing normalization tests.

cmd/server/tests/media_item_isbn_test.go

@@ -115,6 +115,61 @@ func TestMediaItemISBNNormalization(t *testing.T) {
 			input:    "0-12345-678-X",
 			expected: "012345678X",
 		},
+		{
+			name:     "ISBN-10 converts to ISBN-13",
+			input:    "0-306-40615-2",
+			expected: "9780306406157",
+		},
+		{
+			name:     "ISBN-13 with trailing hyphen",
+			input:    "978030640615-7-",
+			expected: "9780306406157",
+		},
+		{
+			name:     "ISBN-10 with trailing hyphen",
+			input:    "030640615-2-",
+			expected: "0306406152",
+		},
+		{
+			name:     "empty string converts to empty string",
+			input:    "",
+			expected: "",
+		},
+		{
+			name:     "only hyphens converts to empty string",
+			input:    "---",
+			expected: "",
+		},
+		{
+			name:     "only spaces converts to empty string",
+			input:    "   ",
+			expected: "",
+		},
+		{
+			name:     "ISBN-10 with X converts to ISBN-13",
+			input:    "0-596-00965-X",
+			expected: "9780596009656",
+		},
+		{
+			name:     "ISBN-13 with leading/trailing hyphens",
+			input:    "-978-0-306-40615-7-",
+			expected: "9780306406157",
+		},
+		{
+			name:     "ISBN-13 preserves X",
+			input:    "978-0-596-00965-X",
+			expected: "9780596009652",
+		},
+		{
+			name:     "ISBN-13 preserves case",
+			input:    "978-0-306-40615-7",
+			expected: "9780306406157",
+		},
+		{
+			name:     "ISBN-13 lowercase preserved",
+			input:    "978-0-306-40615-7",
+			expected: "9780306406157",
+		},
 	}
 
 	for _, tc := range testCases {