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

internal/store/postgres/peer_username.go

@@ -65,6 +65,21 @@ func getPeerUsernameOwner(ctx context.Context, db sqlcgen.DBTX, usernameLower st
 	return owner, true, nil
 }
 
+// usernameReservedTx reports whether a name is on the operator blocklist. It is
+// consulted before every editable-username write and before a collectible mint.
+func usernameReservedTx(ctx context.Context, db sqlcgen.DBTX, usernameLower string) (bool, error) {
+	if usernameLower == "" {
+		return false, nil
+	}
+	var exists bool
+	if err := db.QueryRow(ctx,
+		`SELECT EXISTS (SELECT 1 FROM reserved_usernames WHERE username_lower = $1)`,
+		usernameLower).Scan(&exists); err != nil {
+		return false, fmt.Errorf("check reserved username: %w", err)
+	}
+	return exists, nil
+}
+
 func peerUsernameAvailable(ctx context.Context, db sqlcgen.DBTX, usernameLower, peerType string, peerID int64) (bool, error) {
 	owner, found, err := getPeerUsernameOwner(ctx, db, usernameLower, false)
 	if err != nil || !found {
@@ -115,6 +130,11 @@ WHERE peer_type = $1
 // otherwise account.updateUsername would silently release a minted asset.
 func replacePeerUsernameTx(ctx context.Context, tx pgx.Tx, peerType string, peerID int64, username, usernameLower string) error {
 	if usernameLower != "" {
+		if reserved, err := usernameReservedTx(ctx, tx, usernameLower); err != nil {
+			return err
+		} else if reserved {
+			return domain.ErrUsernameOccupied
+		}
 		owner, found, err := getPeerUsernameOwner(ctx, tx, usernameLower, true)
 		if err != nil {
 			return err

internal/store/postgres/reserved_username.go

@@ -0,0 +1,99 @@
+package postgres
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"telesrv/internal/domain"
+	"telesrv/internal/store/postgres/sqlcgen"
+)
+
+// ReservedUsernameStore is the operator username blocklist backed by the
+// reserved_usernames table.
+type ReservedUsernameStore struct {
+	db sqlcgen.DBTX
+}
+
+// NewReservedUsernameStore builds the store on a pgx pool or transaction.
+func NewReservedUsernameStore(db sqlcgen.DBTX) *ReservedUsernameStore {
+	return &ReservedUsernameStore{db: db}
+}
+
+func (s *ReservedUsernameStore) IsReserved(ctx context.Context, usernameLower string) (bool, error) {
+	usernameLower = strings.ToLower(strings.TrimSpace(usernameLower))
+	if usernameLower == "" {
+		return false, nil
+	}
+	var exists bool
+	if err := s.db.QueryRow(ctx,
+		`SELECT EXISTS (SELECT 1 FROM reserved_usernames WHERE username_lower = $1)`,
+		usernameLower).Scan(&exists); err != nil {
+		return false, fmt.Errorf("check reserved username: %w", err)
+	}
+	return exists, nil
+}
+
+func (s *ReservedUsernameStore) ReserveUsername(ctx context.Context, username, reason, actor string) (bool, error) {
+	username = strings.TrimSpace(username)
+	lower := strings.ToLower(username)
+	if lower == "" {
+		return false, domain.ErrUsernameInvalid
+	}
+	tag, err := s.db.Exec(ctx, `
+INSERT INTO reserved_usernames (username_lower, username, reason, actor)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (username_lower) DO NOTHING`, lower, username, reason, actor)
+	if err != nil {
+		return false, fmt.Errorf("reserve username: %w", err)
+	}
+	return tag.RowsAffected() > 0, nil
+}
+
+func (s *ReservedUsernameStore) UnreserveUsername(ctx context.Context, username string) (bool, error) {
+	lower := strings.ToLower(strings.TrimSpace(username))
+	if lower == "" {
+		return false, domain.ErrUsernameInvalid
+	}
+	tag, err := s.db.Exec(ctx, `DELETE FROM reserved_usernames WHERE username_lower = $1`, lower)
+	if err != nil {
+		return false, fmt.Errorf("unreserve username: %w", err)
+	}
+	return tag.RowsAffected() > 0, nil
+}
+
+func (s *ReservedUsernameStore) ReservedUsernames(ctx context.Context, filter domain.ReservedUsernameFilter) ([]domain.ReservedUsername, error) {
+	limit := filter.Limit
+	if limit <= 0 || limit > 500 {
+		limit = 100
+	}
+	offset := filter.Offset
+	if offset < 0 {
+		offset = 0
+	}
+	args := []any{limit, offset}
+	where := ""
+	if q := strings.ToLower(strings.TrimSpace(filter.Query)); q != "" {
+		args = append(args, q+"%")
+		where = "WHERE username_lower LIKE $3"
+	}
+	rows, err := s.db.Query(ctx, `
+SELECT username, reason, actor, created_at
+FROM reserved_usernames
+`+where+`
+ORDER BY created_at DESC, username_lower
+LIMIT $1 OFFSET $2`, args...)
+	if err != nil {
+		return nil, fmt.Errorf("list reserved usernames: %w", err)
+	}
+	defer rows.Close()
+	out := make([]domain.ReservedUsername, 0, limit)
+	for rows.Next() {
+		var item domain.ReservedUsername
+		if err := rows.Scan(&item.Username, &item.Reason, &item.Actor, &item.CreatedAt); err != nil {
+			return nil, fmt.Errorf("scan reserved username: %w", err)
+		}
+		out = append(out, item)
+	}
+	return out, rows.Err()
+}