Stage 17 round 6 (#4/#5/#6 backend): per-game draft store + conflict reset

Foundation for persisting a player's client-side composition: a game_drafts table
(game_id, account_id, rack_order, board_tiles jsonb) with raw-SQL store/service methods —
GetDraft/SaveDraft (seated-player check) and, on every committed move, clearing the actor's
own draft and resetting any opponent's board draft whose cell the play overlapped (the
draft can no longer be placed; the rack order is kept). Integration tests cover the
round-trip, the actor clear, the overlap reset, a non-conflicting survival, and the
outsider rejection. The gateway op slice + UI wiring follow.

backend/internal/postgres/migrations/00011_game_drafts.sql

@@ -0,0 +1,21 @@
+-- +goose Up
+-- Stage 17: a per-(game, account) draft the server persists across reloads and devices —
+-- the player's preferred rack tile order (#4) and the tiles they have laid on the board but
+-- not yet submitted (#5/#6). board_tiles is reset when an opponent's committed move overlaps
+-- one of its cells (the draft can no longer be placed). Queried with raw SQL, so no
+-- generated jet code is needed.
+SET search_path = backend, pg_catalog;
+
+CREATE TABLE game_drafts (
+    game_id     uuid        NOT NULL REFERENCES games (game_id) ON DELETE CASCADE,
+    account_id  uuid        NOT NULL REFERENCES accounts (account_id) ON DELETE CASCADE,
+    rack_order  text        NOT NULL DEFAULT '',
+    board_tiles jsonb       NOT NULL DEFAULT '[]',
+    updated_at  timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (game_id, account_id)
+);
+
+-- +goose Down
+SET search_path = backend, pg_catalog;
+
+DROP TABLE game_drafts;