feat(subsonic): getUser + envelope-shaped 404 for /rest/*

Feishin's first-login flow hits getUser to discover the authenticated
identity's roles. We never registered the route, so chi returned a
plain-text 404 — Feishin's parser treats anything non-Subsonic as a
generic auth failure ("Failed to log in"), masking the real cause.

- Implement /rest/getUser with the full role bag. Admins get every
  role; non-admins get the play-music subset. Single-user M1 means
  cross-user lookups by admins return the caller's roles for now;
  revisit when user management lands.
- Set sub.NotFound on /rest/* to emit a Subsonic envelope (code 0,
  "Method not implemented") instead of plain text. Any future client
  probing an unimplemented endpoint now sees a parseable failure.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/subsonic/user_test.go

@@ -0,0 +1,78 @@
+package subsonic
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+func TestHandleGetUser_Self(t *testing.T) {
+	user := dbq.User{Username: "alice", IsAdmin: false}
+	w, body := callGetUser(t, user, "")
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", w.Code)
+	}
+	if body.Resp.Status != "ok" {
+		t.Errorf("status = %q, want ok", body.Resp.Status)
+	}
+	if body.Resp.User.Username != "alice" {
+		t.Errorf("username = %q, want alice", body.Resp.User.Username)
+	}
+	if body.Resp.User.AdminRole {
+		t.Error("non-admin should not have admin role")
+	}
+	if !body.Resp.User.StreamRole {
+		t.Error("stream role must be on so clients enable playback")
+	}
+}
+
+func TestHandleGetUser_AdminGetsAdminRole(t *testing.T) {
+	admin := dbq.User{Username: "admin", IsAdmin: true}
+	_, body := callGetUser(t, admin, "")
+	if !body.Resp.User.AdminRole || !body.Resp.User.SettingsRole {
+		t.Errorf("admin/settings roles = %v/%v, want true/true",
+			body.Resp.User.AdminRole, body.Resp.User.SettingsRole)
+	}
+}
+
+func TestHandleGetUser_NonAdminCannotLookupOthers(t *testing.T) {
+	user := dbq.User{Username: "alice", IsAdmin: false}
+	_, body := callGetUser(t, user, "bob")
+	if body.Resp.Status != "failed" || body.Resp.Error == nil ||
+		body.Resp.Error.Code != ErrNotAuthorized {
+		t.Errorf("expected ErrNotAuthorized, got status=%q err=%+v",
+			body.Resp.Status, body.Resp.Error)
+	}
+}
+
+type getUserPayload struct {
+	Resp struct {
+		Status string `json:"status"`
+		Error  *Error `json:"error"`
+		User   User   `json:"user"`
+	} `json:"subsonic-response"`
+}
+
+func callGetUser(t *testing.T, user dbq.User, requestedUsername string) (*httptest.ResponseRecorder, getUserPayload) {
+	t.Helper()
+	url := "/rest/getUser.view?f=json"
+	if requestedUsername != "" {
+		url += "&username=" + requestedUsername
+	}
+	req := httptest.NewRequest("GET", url, nil)
+	req = req.WithContext(context.WithValue(req.Context(), userCtxKey, user))
+	w := httptest.NewRecorder()
+	handleGetUser(w, req)
+
+	var body getUserPayload
+	if err := json.NewDecoder(strings.NewReader(w.Body.String())).Decode(&body); err != nil {
+		t.Fatalf("decode response: %v\nbody=%s", err, w.Body.String())
+	}
+	return w, body
+}