feat: cleanup stale concurrency slots on startup

When the service restarts, concurrency slots from the old process
remain in Redis, causing phantom occupancy. On startup, scan all
concurrency sorted sets and remove members with non-current process
prefix, then clear orphaned wait queue counters.

Uses Go-side SCAN to discover keys (compatible with Redis client
prefix hooks in tests), then passes them to a Lua script for
atomic member-level cleanup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/testutil/stubs.go

@@ -76,6 +76,9 @@ func (c StubConcurrencyCache) GetAccountConcurrencyBatch(_ context.Context, acco
 func (c StubConcurrencyCache) CleanupExpiredAccountSlots(_ context.Context, _ int64) error {
 	return nil
 }
+func (c StubConcurrencyCache) CleanupStaleProcessSlots(_ context.Context, _ string) error {
+	return nil
+}
 
 // ============================================================
 // StubGatewayCache — service.GatewayCache 的空实现