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.

cmd/telesrv-admin/web/src/routing.ts

@@ -20,6 +20,7 @@ export function routeTitle(pathname: string): string {
   if (pathname.startsWith("/bot-verification")) return "Third-party verification";
   if (pathname.startsWith("/verification")) return "Official Verification";
   if (pathname.startsWith("/collectible-usernames")) return "Collectible Usernames";
+  if (pathname.startsWith("/reserved-usernames")) return "Reserved Usernames";
   if (pathname.startsWith("/storage")) return "Storage";
   if (pathname.startsWith("/accounts/shared-devices")) return "Shared Devices";
   if (pathname.startsWith("/accounts")) return "Accounts";