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

backend/internal/server/handlers_friends.go

@@ -48,9 +48,12 @@ type redeemResultDTO struct {
 	Friend accountRefDTO `json:"friend"`
 }
 
-// targetIDRequest carries a single counterpart account id.
+// targetIDRequest carries a single counterpart account id. GameID is set only by an
+// in-game block (so a disguised-robot opponent can be recorded as a per-game block); the
+// other target paths leave it empty.
 type targetIDRequest struct {
 	AccountID string `json:"account_id"`
+	GameID    string `json:"game_id"`
 }
 
 // friendRespondRequest accepts or declines a pending request from a requester.