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/httpapi/middleware.go

@@ -79,3 +79,34 @@ func currentUserFromContext(r *http.Request) auth.User {
 	}
 	return user
 }
+
+func (a app) requireSubsonicAuth(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		user, err := a.auth.CurrentUserBySubsonicAuth(
+			r.Context(),
+			r.URL.Query().Get("u"),
+			r.URL.Query().Get("p"),
+			r.URL.Query().Get("t"),
+			r.URL.Query().Get("s"),
+		)
+		if err != nil {
+			writeJSON(w, http.StatusUnauthorized, map[string]any{
+				"subsonic-response": map[string]any{
+					"status":        "failed",
+					"version":       "1.16.1",
+					"type":          "temporserv",
+					"serverVersion": "0.1.0",
+					"openSubsonic":  true,
+					"error": map[string]any{
+						"code":    40,
+						"message": "Wrong username or password",
+					},
+				},
+			})
+			return
+		}
+
+		ctx := context.WithValue(r.Context(), currentUserKey, user)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}