usernames: operator reserved-username blocklist

A plain blocklist for names like @support - separate from the collectible
system, so a reservation has no owner, no price and no "bought on Fragment"
badge.

- reserved_usernames table + migration.
- Enforced in replacePeerUsernameTx (the single editable-username write point:
  account.updateUsername, channels.updateUsername, @BotFather /setusername) and
  in the collectible mint path; a reserved name returns USERNAME_OCCUPIED.
- admin.Service: ReserveUsername / UnreserveUsername (journalled commands) and
  the ReservedUsernames listing.
- adminapi: /v1/reserved-usernames{,/reserve,/unreserve}.
- telesrv-admin panel + a "Reserved Usernames" page in the web UI (dist rebuilt).
- Postgres and in-memory store implementations; the memory registry gains an
  optional reserved-name check so tests exercise the same rule.

internal/store/memory/collectible_username.go

@@ -55,6 +55,24 @@ type CollectibleUsernameStore struct {
 	transfers map[int64][]domain.CollectibleUsernameTransfer
 	// commands maps a provenance command key onto the asset it touched.
 	commands map[string]int64
+	// reserved, when set, is the operator blocklist consulted before a name is
+	// assigned to an editable slot or minted, mirroring the PostgreSQL checks.
+	reserved *ReservedUsernameStore
+}
+
+// WithReservedUsernames wires the operator blocklist into the registry so a
+// reserved name is refused, matching PostgreSQL.
+func (s *CollectibleUsernameStore) WithReservedUsernames(reserved *ReservedUsernameStore) *CollectibleUsernameStore {
+	s.reserved = reserved
+	return s
+}
+
+func (s *CollectibleUsernameStore) nameReservedLocked(usernameLower string) bool {
+	if s.reserved == nil {
+		return false
+	}
+	r, _ := s.reserved.IsReserved(context.Background(), usernameLower)
+	return r
 }
 
 // collectibleRegistryRow is one peer_usernames row: the owning peer plus the
@@ -101,6 +119,9 @@ func (s *CollectibleUsernameStore) SetEditableUsername(_ context.Context, peer d
 		return false, domain.ErrUsernameInvalid
 	}
 	key := strings.ToLower(username)
+	if s.nameReservedLocked(key) {
+		return false, domain.ErrUsernameOccupied
+	}
 	if existing, ok := s.registry[key]; ok {
 		if existing.peer == peer && existing.row.Editable {
 			if existing.row.Username == username {
@@ -313,6 +334,9 @@ func (s *CollectibleUsernameStore) MintCollectibleUsername(_ context.Context, re
 	if _, ok := s.registry[key]; ok {
 		return domain.CollectibleUsername{}, false, domain.ErrUsernameOccupied
 	}
+	if s.nameReservedLocked(key) {
+		return domain.CollectibleUsername{}, false, domain.ErrUsernameOccupied
+	}
 	now := time.Now().UTC()
 	purchaseDate := req.PurchaseDate
 	if purchaseDate.IsZero() {