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/handlers/accounts_test.go

@@ -220,6 +220,29 @@ func TestAdminUploadBypassesMaxUploadSize(t *testing.T) {
 	}
 }
 
+func TestUnlimitedAnonymousUploadPolicyUsesNegativeOne(t *testing.T) {
+	app, cleanup := newTestApp(t)
+	defer cleanup()
+
+	policy, err := app.settingsService.UploadPolicy()
+	if err != nil {
+		t.Fatalf("UploadPolicy returned error: %v", err)
+	}
+	policy.AnonymousMaxUploadMB = -1
+	policy.AnonymousDailyUploadMB = -1
+	if err := app.settingsService.UpdateUploadPolicy(policy); err != nil {
+		t.Fatalf("UpdateUploadPolicy returned error: %v", err)
+	}
+
+	request := multipartUploadRequest(t, "/api/v1/upload", "file", "large.txt", strings.Repeat("x", int(app.uploadService.MaxUploadSize())+1))
+	request.Header.Set("Accept", "application/json")
+	response := httptest.NewRecorder()
+	app.Upload(response, request)
+	if response.Code != http.StatusCreated {
+		t.Fatalf("unlimited anonymous upload status = %d, body = %s", response.Code, response.Body.String())
+	}
+}
+
 func TestAnonymousUploadDisabled(t *testing.T) {
 	app, cleanup := newTestApp(t)
 	defer cleanup()