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/api.ts

@@ -22,6 +22,7 @@ import type {
   ChannelListResponse,
   CollectibleUsernameDetail,
   CollectibleUsernameListResponse,
+  ReservedUsernameListResponse,
   CommandResult,
   GroupMessageDetail,
   GroupMessageListResponse,
@@ -163,6 +164,8 @@ export const api = {
     request<CollectibleUsernameListResponse>(`/api/collectible-usernames?${params.toString()}`),
   collectibleUsername: (id: string) =>
     request<CollectibleUsernameDetail>(`/api/collectible-usernames/${encodeURIComponent(id)}`),
+  reservedUsernames: (params: URLSearchParams) =>
+    request<ReservedUsernameListResponse>(`/api/reserved-usernames?${params.toString()}`),
   dashboard: () => request<DashboardResponse>("/api/dashboard"),
   storageStats: () => request<StorageStatsResponse>("/api/storage/stats"),
   storageAccounts: (params: URLSearchParams) =>