feat(config): allow -1 to represent unlimited upload limits

Introduce support for configuring unlimited upload limits by allowing -1
as a valid value for anonymous and user upload MB limits.

Changes include:
- Added `envMegabytesLimitFloat` and helper functions to parse and validate limits where -1 is allowed.
- Updated validation logic to accept -1 for `AnonymousMaxUploadMB`, `AnonymousDailyUploadMB`, and `UserDailyUploadMB`.
- Added a test case to verify unlimited upload policy behavior.

backend/libs/services/settings_test.go

@@ -117,6 +117,30 @@ func TestSettingsRejectInvalidMegabytes(t *testing.T) {
 	}
 }
 
+func TestUploadPolicyAllowsNegativeOneForUnlimitedUploadLimits(t *testing.T) {
+	settings := newTestSettingsService(t)
+	policy, err := settings.UploadPolicy()
+	if err != nil {
+		t.Fatalf("UploadPolicy returned error: %v", err)
+	}
+	policy.AnonymousMaxUploadMB = -1
+	policy.AnonymousDailyUploadMB = -1
+	policy.UserDailyUploadMB = -1
+	if err := settings.UpdateUploadPolicy(policy); err != nil {
+		t.Fatalf("UpdateUploadPolicy rejected -1 unlimited upload limits: %v", err)
+	}
+	next, err := settings.UploadPolicy()
+	if err != nil {
+		t.Fatalf("UploadPolicy returned error: %v", err)
+	}
+	if next.AnonymousMaxUploadMB != -1 || next.AnonymousDailyUploadMB != -1 || next.UserDailyUploadMB != -1 {
+		t.Fatalf("unlimited upload limits were not persisted: %+v", next)
+	}
+	if got := FormatMegabytesLabel(-1); got != "unlimited" {
+		t.Fatalf("FormatMegabytesLabel(-1) = %q, want unlimited", got)
+	}
+}
+
 func TestDailyUsageAndCleanup(t *testing.T) {
 	settings := newTestSettingsService(t)
 	now := time.Date(2026, 5, 30, 12, 0, 0, 0, time.UTC)