admin: bypass the reserved-username blocklist at write time too

UpdateUsernameAdmin already skipped the reserved-word check in the
availability lookup, but UserStore.UpdateUsername's own write path
(replacePeerUsernameTx / CollectibleUsernameStore.SetEditableUsername)
enforces the same operator blocklist a second time, independently and
unconditionally. That second check is what was still rejecting an admin
handing out a word they'd deliberately reserved, with "username occupied".

Add UpdateUsernameAdmin/SetEditableUsernameAdmin bypass variants down the
write path (postgres and memory) and route users.Service's actual write
through them when the availability check was already bypassed.

internal/app/users/service.go

@@ -48,6 +48,24 @@ type usernameAvailabilityStore interface {
 	CheckUsername(ctx context.Context, userID int64, username string) (bool, error)
 }
 
+// adminUsernameAvailabilityStore is usernameAvailabilityStore's admin-bypass
+// counterpart: CheckUsernameAdmin skips the operator reserved-username
+// blocklist, so UpdateUsernameAdmin can hand a deliberately reserved word to
+// a specific account instead of that same reservation blocking the operator's
+// own assignment.
+type adminUsernameAvailabilityStore interface {
+	CheckUsernameAdmin(ctx context.Context, userID int64, username string) (bool, error)
+}
+
+// adminUsernameStore is store.UserStore's admin-bypass counterpart for the
+// actual write: UpdateUsername's own write path enforces the reserved-word
+// blocklist a second time (independent of the availability check), so
+// bypassing only checkUsernameAvailable isn't enough -- the write itself
+// needs UpdateUsernameAdmin too.
+type adminUsernameStore interface {
+	UpdateUsernameAdmin(ctx context.Context, userID int64, username string) (domain.User, error)
+}
+
 type moderationFlagAudienceStore interface {
 	ModerationFlagAudience(ctx context.Context, userID int64, limit int) ([]int64, error)
 }
@@ -318,6 +336,17 @@ func (s *Service) checkUsernameAvailable(ctx context.Context, selfID int64, user
 	return !found || u.ID == selfID, nil
 }
 
+// checkUsernameAvailableAdmin is checkUsernameAvailable without the operator
+// reserved-username blocklist. Falls back to checkUsernameAvailable (which
+// does enforce it) if the store doesn't implement the admin-bypass method --
+// availability checking still works, just without the bypass.
+func (s *Service) checkUsernameAvailableAdmin(ctx context.Context, selfID int64, username string) (bool, error) {
+	if checker, ok := s.users.(adminUsernameAvailabilityStore); ok {
+		return checker.CheckUsernameAdmin(ctx, selfID, username)
+	}
+	return s.checkUsernameAvailable(ctx, selfID, username)
+}
+
 // UpdateUsername 修改当前用户的主 username（self-service）。空字符串表示删除 username。
 func (s *Service) UpdateUsername(ctx context.Context, userID int64, username string) (domain.User, error) {
 	return s.updateUsername(ctx, userID, username, true)
@@ -350,7 +379,15 @@ func (s *Service) updateUsername(ctx context.Context, userID int64, username str
 		if !validUsername(username) || (enforceReserved && s.reserved.Contains(username)) {
 			return domain.User{}, domain.ErrUsernameInvalid
 		}
-		ok, err := s.checkUsernameAvailable(ctx, self.ID, username)
+		var (
+			ok  bool
+			err error
+		)
+		if enforceReserved {
+			ok, err = s.checkUsernameAvailable(ctx, self.ID, username)
+		} else {
+			ok, err = s.checkUsernameAvailableAdmin(ctx, self.ID, username)
+		}
 		if err != nil {
 			return domain.User{}, err
 		}
@@ -358,7 +395,14 @@ func (s *Service) updateUsername(ctx context.Context, userID int64, username str
 			return domain.User{}, domain.ErrUsernameOccupied
 		}
 	}
-	u, err := s.users.UpdateUsername(ctx, self.ID, username)
+	var u domain.User
+	if enforceReserved {
+		u, err = s.users.UpdateUsername(ctx, self.ID, username)
+	} else if admin, ok := s.users.(adminUsernameStore); ok {
+		u, err = admin.UpdateUsernameAdmin(ctx, self.ID, username)
+	} else {
+		u, err = s.users.UpdateUsername(ctx, self.ID, username)
+	}
 	if err != nil {
 		return domain.User{}, err
 	}

internal/app/users/service_test.go

@@ -151,6 +151,39 @@ func TestServiceUpdateUsernameAdminBypassesReserved(t *testing.T) {
 	}
 }
 
+// TestServiceUpdateUsernameAdminBypassesOperatorReservedTable locks in that
+// UpdateUsernameAdmin also bypasses the *database-backed* reserved-usernames
+// blocklist (the admin console's own Reserved Usernames feature), not just
+// config.ReservedUsernames -- an operator who deliberately reserves a word
+// via that feature must still be able to hand it to a specific account,
+// instead of their own reservation blocking them with "username occupied".
+func TestServiceUpdateUsernameAdminBypassesOperatorReservedTable(t *testing.T) {
+	ctx := context.Background()
+	userStore := memory.NewUserStore()
+	reserved := memory.NewReservedUsernameStore()
+	registry := memory.NewCollectibleUsernameStore().WithReservedUsernames(reserved)
+	userStore.AttachUsernameRegistry(registry)
+	if _, err := reserved.ReserveUsername(ctx, "durov", "brand protection", "operator"); err != nil {
+		t.Fatalf("reserve username: %v", err)
+	}
+	target, err := userStore.Create(ctx, domain.User{AccessHash: 1, Phone: "15550000006", FirstName: "Target"})
+	if err != nil {
+		t.Fatalf("create target: %v", err)
+	}
+	svc := NewService(userStore)
+
+	if _, err := svc.UpdateUsername(ctx, target.ID, "durov"); !errors.Is(err, domain.ErrUsernameOccupied) {
+		t.Fatalf("self-service claim of operator-reserved username err = %v, want username occupied", err)
+	}
+	u, err := svc.UpdateUsernameAdmin(ctx, target.ID, "durov")
+	if err != nil {
+		t.Fatalf("admin claim of operator-reserved username: %v", err)
+	}
+	if u.Username != "durov" {
+		t.Fatalf("admin claim of operator-reserved username: got username %q, want %q", u.Username, "durov")
+	}
+}
+
 // marksbotOverrideStore wraps memory.UserStore to serve domain.VerifierBotUser()
 // for a fixed username lookup, since memory.UserStore.Create always assigns an
 // id from its own auto-increment sequence and can never produce the fixed