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/postgres/collectible_username.go

@@ -206,6 +206,11 @@ func (s *CollectibleUsernameStore) MintCollectibleUsername(ctx context.Context,
 		} else if found {
 			return domain.ErrUsernameOccupied
 		}
+		if reserved, err := usernameReservedTx(ctx, tx, usernameLower); err != nil {
+			return err
+		} else if reserved {
+			return domain.ErrUsernameOccupied
+		}
 		var existing int64
 		switch err := tx.QueryRow(ctx, `
 SELECT id FROM collectible_usernames