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/main.go

@@ -1491,6 +1491,7 @@ func run(logger *zap.Logger) error {
 	// Collectible (NFT) usernames are an optional read model projected at the
 	// protocol edge.
 	collectibleUsernameStore := postgres.NewCollectibleUsernameStore(pool)
+	reservedUsernameStore := postgres.NewReservedUsernameStore(pool)
 	usernamesService := usernamesapp.NewService(
 		usernamesapp.WithRegistryStore(collectibleUsernameStore),
 		usernamesapp.WithCollectibleStore(collectibleUsernameStore),
@@ -1704,6 +1705,7 @@ func run(logger *zap.Logger) error {
 		Emoji:                  filesService,
 		Moderation:             moderationService,
 		Usernames:              usernamesService,
+		ReservedUsernames:      reservedUsernameStore,
 		Verification:           verificationService,
 		BotVerification:        botVerificationService,
 		Account:                accountService,