fix(billing): treat nil rate limit window as expired to prevent usage accumulation

When Redis cache is populated from DB with a NULL window_1d_start, the
Lua increment script only updates usage counters without setting window
timestamps. IsWindowExpired(nil) previously returned false, so the
accumulated usage was never reset across time windows, effectively
turning usage_1d into a lifetime counter. Once this exceeded
rate_limit_1d the key was incorrectly blocked with "日限额已用完".

Fixes Wei-Shaw/sub2api#1022

backend/internal/service/api_key.go

@@ -22,8 +22,9 @@ const (
 )
 
 // IsWindowExpired returns true if the window starting at windowStart has exceeded the given duration.
+// A nil windowStart is treated as expired — no initialized window means any accumulated usage is stale.
 func IsWindowExpired(windowStart *time.Time, duration time.Duration) bool {
-	return windowStart != nil && time.Since(*windowStart) >= duration
+	return windowStart == nil || time.Since(*windowStart) >= duration
 }
 
 type APIKey struct {