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/auth_test.go

@@ -205,6 +205,26 @@ func TestAPITokenScopedToOwnerAndDisabledUser(t *testing.T) {
 	}
 }
 
+func TestUserPolicyAllowsNegativeOneForUnlimitedUploadLimits(t *testing.T) {
+	auth := newTestAuthService(t)
+	user, err := auth.CreateBootstrapUser("daniel", "daniel@example.test", "password123")
+	if err != nil {
+		t.Fatalf("CreateBootstrapUser returned error: %v", err)
+	}
+
+	unlimited := -1.0
+	if err := auth.SetUserPolicy(user.ID, UserPolicy{MaxUploadMB: &unlimited, DailyUploadMB: &unlimited}); err != nil {
+		t.Fatalf("SetUserPolicy rejected -1 unlimited upload limits: %v", err)
+	}
+	updated, err := auth.UserByID(user.ID)
+	if err != nil {
+		t.Fatalf("UserByID returned error: %v", err)
+	}
+	if updated.Policy.MaxUploadMB == nil || *updated.Policy.MaxUploadMB != -1 || updated.Policy.DailyUploadMB == nil || *updated.Policy.DailyUploadMB != -1 {
+		t.Fatalf("unlimited policy was not persisted: %+v", updated.Policy)
+	}
+}
+
 func newTestAuthService(t *testing.T) *AuthService {
 	t.Helper()
 	root := t.TempDir()