fix(matchmaking): re-enqueue opens a new game, not the caller's own

A second "random opponent" enqueue with the same variant and per-turn
rule, while the caller's first game was still open (awaiting an
opponent), returned that same open game, so a player could never start a
fresh random game while one was still searching.

Drop the own-open short-circuit (step 1) in store.OpenOrJoin: a
re-enqueue now joins another player's open game or opens a fresh one.
Accumulation stays bounded by MaxActiveQuickGames, which counts open
games. Update the matchmaker/service/store doc comments and
ARCHITECTURE.md, and flip the pinning test to assert the new behavior.

backend/internal/lobby/matchmaker.go

@@ -85,9 +85,10 @@ type EnqueueResult struct {
 
 // Enqueue resolves an auto-match request for accountID under variant and the per-turn
 // word rule (multipleWords) into the game they enter immediately — a freshly opened
-// game awaiting an opponent, the caller's own still-open game (a re-enqueue is
-// idempotent), or another player's open game they just joined. When the caller joins
-// an existing game, opponent_joined is pushed to that game's waiting starter.
+// game awaiting an opponent, or another player's open game they just joined. A
+// re-enqueue while already waiting opens another game rather than returning the
+// caller's own. When the caller joins an existing game, opponent_joined is pushed to
+// that game's waiting starter.
 func (m *Matchmaker) Enqueue(ctx context.Context, accountID uuid.UUID, variant engine.Variant, multipleWords bool) (EnqueueResult, error) {
 	g, joined, err := m.games.OpenOrJoin(ctx, accountID, autoMatchParams(variant, multipleWords), m.openDeadline())
 	if err != nil {