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/postgres/migrations/00011_robot_blocks.sql

@@ -0,0 +1,27 @@
+-- +goose Up
+-- Per-game blocks of a disguised-robot opponent. A disguised robot is a shared pool
+-- account reused across games under different per-game names, so it must never go into
+-- `blocks` (that would make the same robot look blocked in the blocker's other games under
+-- other names, leaking that it is a bot, and show its pool name instead of the one seen).
+-- Each such block is recorded here against the specific game + seat, snapshotting the name
+-- the player saw, so the blocked list shows it as a distinct personality and the in-game
+-- card re-marks that one seat. The real robot account is never blocked, so the matchmaker
+-- leaves robots free. Unblocking deletes the row.
+SET search_path = backend, pg_catalog;
+
+CREATE TABLE robot_blocks (
+    id           uuid        PRIMARY KEY,
+    blocker_id   uuid        NOT NULL REFERENCES accounts (account_id) ON DELETE CASCADE,
+    game_id      uuid        NOT NULL REFERENCES games (game_id) ON DELETE CASCADE,
+    seat         smallint    NOT NULL,
+    robot_id     uuid        NOT NULL REFERENCES accounts (account_id) ON DELETE CASCADE,
+    display_name text        NOT NULL,
+    created_at   timestamptz NOT NULL DEFAULT now(),
+    UNIQUE (blocker_id, game_id, seat)
+);
+
+CREATE INDEX robot_blocks_blocker_idx ON robot_blocks (blocker_id);
+
+-- +goose Down
+SET search_path = backend, pg_catalog;
+DROP TABLE robot_blocks;