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.

deploy/migrations/20260909190000_reserved_usernames.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS public.reserved_usernames;

deploy/migrations/20260909190000_reserved_usernames.up.sql

@@ -0,0 +1,17 @@
+-- Operator-maintained username blocklist. A name listed here cannot be taken as
+-- an editable username by any peer (account.updateUsername, channels.updateUsername,
+-- @BotFather /setusername, or the admin set-username actions). It is a plain
+-- blocklist: no owner, no price, no Fragment collectible badge.
+
+CREATE TABLE public.reserved_usernames (
+    username_lower text PRIMARY KEY CHECK (
+        username_lower <> '' AND username_lower = lower(username_lower)
+    ),
+    username   text NOT NULL,
+    reason     text NOT NULL DEFAULT '' CHECK (octet_length(reason) <= 512),
+    actor      text NOT NULL DEFAULT '' CHECK (octet_length(actor) <= 256),
+    created_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX reserved_usernames_created_at_idx
+    ON public.reserved_usernames (created_at DESC, username_lower);