Add GET /api/v1/character/details with each image's change time

Returns every character image with when it was last changed, in Unix seconds,
so clients can cache the images and download only the ones that changed.

internal/backend/characters.go

@@ -37,6 +37,33 @@ func (c *CharacterService) List() ([]string, error) {
 	return characters, nil
 }
 
+// CharacterDetails is a character image with the time its file was last changed, so a
+// client that caches images knows when to download one again.
+type CharacterDetails struct {
+	Name     string `json:"name" doc:"File name of the character image"`
+	Modified int64  `json:"modified" doc:"When the image was last changed, in Unix seconds"`
+}
+
+// ListDetails returns all character images with the time each was last changed.
+func (c *CharacterService) ListDetails() ([]CharacterDetails, error) {
+	entries, err := fs.ReadDir(c.root.FS(), ".")
+	if err != nil {
+		return nil, err
+	}
+	characters := []CharacterDetails{}
+	for _, entry := range entries {
+		if entry.IsDir() || !isImage(entry.Name()) {
+			continue
+		}
+		info, err := entry.Info()
+		if err != nil {
+			return nil, err
+		}
+		characters = append(characters, CharacterDetails{Name: entry.Name(), Modified: info.ModTime().Unix()})
+	}
+	return characters, nil
+}
+
 // Open opens the character image with the given file name.
 func (c *CharacterService) Open(name string) (*os.File, error) {
 	if name == "" || name != filepath.Base(name) || !isImage(name) {

internal/backend/characters_test.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"path/filepath"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -31,6 +32,32 @@ func TestCharacterList(t *testing.T) {
 	assert.Equal(t, []string{"Adol.png", "Aloy.JPG"}, characters)
 }
 
+func TestCharacterListDetails(t *testing.T) {
+	c := newCharacterService(t)
+	changed := time.Date(2026, 10, 7, 12, 0, 0, 0, time.UTC)
+	require.NoError(t, c.root.Chtimes("Adol.png", changed, changed))
+
+	characters, err := c.ListDetails()
+	require.NoError(t, err)
+	require.Len(t, characters, 2)
+	assert.Equal(t, CharacterDetails{Name: "Adol.png", Modified: changed.Unix()}, characters[0])
+	assert.Equal(t, "Aloy.JPG", characters[1].Name)
+}
+
+func TestCharacterListDetailsSeesChangedImage(t *testing.T) {
+	c := newCharacterService(t)
+	before, err := c.ListDetails()
+	require.NoError(t, err)
+
+	later := time.Unix(before[0].Modified, 0).Add(time.Hour)
+	require.NoError(t, c.root.Chtimes("Adol.png", later, later))
+
+	after, err := c.ListDetails()
+	require.NoError(t, err)
+	assert.Greater(t, after[0].Modified, before[0].Modified)
+	assert.Equal(t, before[1], after[1], "unchanged images keep their time")
+}
+
 func TestCharacterOpen(t *testing.T) {
 	c := newCharacterService(t)