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 {

backend/internal/service/api_key_rate_limit_test.go

@@ -15,10 +15,10 @@ func TestIsWindowExpired(t *testing.T) {
 		want     bool
 	}{
 		{
-			name:     "nil window start",
+			name:     "nil window start (treated as expired)",
 			start:    nil,
 			duration: RateLimitWindow5h,
-			want:     false,
+			want:     true,
 		},
 		{
 			name:     "active window (started 1h ago, 5h window)",
@@ -113,7 +113,7 @@ func TestAPIKey_EffectiveUsage(t *testing.T) {
 			want7d: 0,
 		},
 		{
-			name: "nil window starts return raw usage",
+			name: "nil window starts return 0 (stale usage reset)",
 			key: APIKey{
 				Usage5h:       5.0,
 				Usage1d:       10.0,
@@ -122,9 +122,9 @@ func TestAPIKey_EffectiveUsage(t *testing.T) {
 				Window1dStart: nil,
 				Window7dStart: nil,
 			},
-			want5h: 5.0,
-			want1d: 10.0,
-			want7d: 50.0,
+			want5h: 0,
+			want1d: 0,
+			want7d: 0,
 		},
 		{
 			name: "mixed: 5h expired, 1d active, 7d nil",
@@ -138,7 +138,7 @@ func TestAPIKey_EffectiveUsage(t *testing.T) {
 			},
 			want5h: 0,
 			want1d: 10.0,
-			want7d: 50.0,
+			want7d: 0,
 		},
 		{
 			name: "zero usage with active windows",
@@ -210,7 +210,7 @@ func TestAPIKeyRateLimitData_EffectiveUsage(t *testing.T) {
 			want7d: 0,
 		},
 		{
-			name: "nil window starts return raw usage",
+			name: "nil window starts return 0 (stale usage reset)",
 			data: APIKeyRateLimitData{
 				Usage5h:       3.0,
 				Usage1d:       8.0,
@@ -219,9 +219,9 @@ func TestAPIKeyRateLimitData_EffectiveUsage(t *testing.T) {
 				Window1dStart: nil,
 				Window7dStart: nil,
 			},
-			want5h: 3.0,
-			want1d: 8.0,
-			want7d: 40.0,
+			want5h: 0,
+			want1d: 0,
+			want7d: 0,
 		},
 	}