fix(scanner): stop splitting multi-byte characters when truncating tags

sanitize() capped tag values with a byte slice, so a value whose limit falls in
the middle of a multi-byte character was stored as invalid UTF-8. defaultMaxTagLength
is 1024, which is not a multiple of 3, so any sufficiently long CJK title hit this.
Only trailing invalid bytes are trimmed, leaving bad bytes elsewhere in the value
untouched.

model/metadata/metadata.go

@@ -9,6 +9,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"github.com/google/uuid"
 	"github.com/navidrome/navidrome/consts"
@@ -366,6 +367,14 @@ func sanitize(filePath string, tagName model.TagName, tag model.TagConf, value s
 	if len(value) > maxLength {
 		log.Trace("Truncated tag value", "tag", tagName, "value", value, "length", len(value), "maxLength", maxLength)
 		value = value[:maxLength]
+		// Drop the trailing partial rune the cut may have left behind. Only trailing
+		// invalid bytes are removed, so pre-existing bad bytes elsewhere are preserved.
+		for len(value) > 0 {
+			if r, size := utf8.DecodeLastRuneInString(value); r != utf8.RuneError || size > 1 {
+				break
+			}
+			value = value[:len(value)-1]
+		}
 	}
 
 	switch tag.Type {

model/metadata/metadata_test.go

@@ -4,6 +4,7 @@ import (
 	"os"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"github.com/navidrome/navidrome/model"
 	"github.com/navidrome/navidrome/model/metadata"
@@ -122,6 +123,27 @@ var _ = Describe("Metadata", func() {
 				Expect(pair[0].Value()).To(HaveLen(1048570))
 			})
 
+			It("should not split a multi-byte character when truncating", func() {
+				// 1024 is not a multiple of 3, so a byte-wise cut lands mid-rune.
+				props.Tags = model.RawTags{
+					"Title": {strings.Repeat("日", 2048)},
+				}
+				md = metadata.New(filePath, props)
+
+				title := md.String(model.TagTitle)
+				Expect(utf8.ValidString(title)).To(BeTrue(), "truncation produced invalid UTF-8")
+				Expect(len(title)).To(BeNumerically("<=", 1024))
+			})
+
+			It("should keep invalid bytes that are not at the truncation point", func() {
+				props.Tags = model.RawTags{
+					"Title": {"a\xffb" + strings.Repeat("c", 2048)},
+				}
+				md = metadata.New(filePath, props)
+
+				Expect(md.String(model.TagTitle)).To(HaveLen(1024))
+			})
+
 			It("should split multiple values", func() {
 				props.Tags = model.RawTags{
 					"Genre": {"Rock/Pop;;Punk"},