feat: add scan status and cover art endpoints

Track scanner status for the web API and Subsonic-compatible scan endpoints, add authenticated cover art serving, and wire album artwork into the web UI. Keep Subsonic auth limited to legacy password mode for now so behavior stays honest with the current bcrypt-based user storage.

internal/subsonic/service.go

@@ -1,6 +1,11 @@
 package subsonic
 
-import "github.com/benya/temporserv/internal/library"
+import (
+	"time"
+
+	"github.com/benya/temporserv/internal/library"
+	"github.com/benya/temporserv/internal/scanner"
+)
 
 type Envelope struct {
 	SubsonicResponse Response `json:"subsonic-response"`
@@ -14,6 +19,8 @@ type Response struct {
 	OpenAPI    bool        `json:"openSubsonic"`
 	Artists    []ArtistRef `json:"artists,omitempty"`
 	RandomSong []SongRef   `json:"randomSongs,omitempty"`
+	ScanStatus *ScanStatus `json:"scanStatus,omitempty"`
+	Error      *ErrorRef   `json:"error,omitempty"`
 }
 
 type ArtistRef struct {
@@ -28,6 +35,20 @@ type SongRef struct {
 	Artist string `json:"artist"`
 }
 
+type ScanStatus struct {
+	Scanning   bool   `json:"scanning"`
+	Count      int    `json:"count"`
+	FolderCount int   `json:"folderCount"`
+	LastError  string `json:"lastError,omitempty"`
+	StartedAt  string `json:"startedAt,omitempty"`
+	FinishedAt string `json:"finishedAt,omitempty"`
+}
+
+type ErrorRef struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+}
+
 func PingResponse() Envelope {
 	return Envelope{
 		SubsonicResponse: Response{
@@ -63,3 +84,33 @@ func RandomSongsResponse(tracks []library.Track) Envelope {
 	}
 	return response
 }
+
+func ScanStatusResponse(status scanner.Status) Envelope {
+	response := PingResponse()
+	response.SubsonicResponse.ScanStatus = &ScanStatus{
+		Scanning:    status.Scanning,
+		Count:       status.Tracks,
+		FolderCount: status.Albums,
+		LastError:   status.LastError,
+		StartedAt:   formatTime(status.StartedAt),
+		FinishedAt:  formatTime(status.FinishedAt),
+	}
+	return response
+}
+
+func ErrorResponse(code int, message string) Envelope {
+	response := PingResponse()
+	response.SubsonicResponse.Status = "failed"
+	response.SubsonicResponse.Error = &ErrorRef{
+		Code:    code,
+		Message: message,
+	}
+	return response
+}
+
+func formatTime(value time.Time) string {
+	if value.IsZero() {
+		return ""
+	}
+	return value.Format(time.RFC3339)
+}