fix(social): robot blocks

Blocking an auto-match opponent who is secretly a pooled robot is recorded  instead in a separate `robot_blocks` table.

Now blocking behaves the same in that game (struck name, hidden composer) and lists the blocked opponent under the name you saw, but is recorded only against that game — the disguise holds, the shared robot is never globally blocked, and the matchmaker keeps pairing you with robots (so you can never block yourself out of opponents).

- the shared robot account is never put in `blocks`
- the matchmaker keeps it free and it is not blocked under its other per-game names
- the blocked list and the in-game card still show it by joining that table; an unblock deletes the row

gateway/internal/transcode/transcode_social.go

@@ -166,7 +166,9 @@ func blocksListHandler(backend *backendclient.Client) Handler {
 func blockAddHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsTargetRequest(req.Payload, 0)
-		if err := backend.Block(ctx, req.UserID, string(in.AccountId())); err != nil {
+		// game_id is set only by an in-game block, so a disguised-robot opponent is recorded
+		// per-game; it is empty for a settings-screen block.
+		if err := backend.Block(ctx, req.UserID, string(in.AccountId()), string(in.GameId())); err != nil {
 			return nil, err
 		}
 		return encodeAck(true), nil