Set the expiry in TestHandleImageEnc_MaxAge inside each subtest (closes #352)

The table set the expiry when it was built. The parallel subtests can wait
more than 10 seconds behind other tests on a busy host, so the 60 second
case saw a max-age under 50. The table now holds the TTL, and each subtest
computes the expiry just before generating its token.

Model: opus-5-5

internal/handlers/image_cache_control_internal_test.go

@@ -185,13 +185,13 @@ func TestHandleImageEnc_MaxAge(t *testing.T) {
 
 	tests := []struct {
 		name        string
-		expires     time.Time
+		ttl         time.Duration
 		wantAtLeast int
 		wantAtMost  int
 	}{
-		{"60 second TTL", time.Now().Add(time.Minute), 50, 60},
-		{"two-year TTL", time.Now().Add(2 * 365 * 24 * time.Hour), 31536000, 31536000},
-		{"no TTL", time.Time{}, 31536000, 31536000},
+		{"60 second TTL", time.Minute, 50, 60},
+		{"two-year TTL", 2 * 365 * 24 * time.Hour, 31536000, 31536000},
+		{"no TTL", 0, 31536000, 31536000},
 	}
 
 	for _, tt := range tests {
@@ -200,6 +200,14 @@ func TestHandleImageEnc_MaxAge(t *testing.T) {
 
 			h, srv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
 
+			// The expiry is set here, not in the table: a parallel subtest
+			// can wait behind other tests for more than the 10 seconds the
+			// 60 second case allows.
+			var expires time.Time
+			if tt.ttl != 0 {
+				expires = time.Now().Add(tt.ttl)
+			}
+
 			token, err := h.encGen.Generate(&imagerequest.Request{
 				Source: imagerequest.Source{
 					Host: signedHost,
@@ -209,7 +217,7 @@ func TestHandleImageEnc_MaxAge(t *testing.T) {
 					Size:   imagerequest.Size{Width: 50, Height: 50},
 					Format: imagerequest.FormatJPEG,
 				},
-			}, tt.expires)
+			}, expires)
 			if err != nil {
 				t.Fatalf("Generate() error = %v", err)
 			}