fix(http): prevent SSRF in outbound webhook and HTTP endpoint requests

Webhook and HTTP message endpoint deliveries both flow through
Postal::HTTP, which parsed the user-supplied URL and connected to its
host with no address validation. An authenticated user could point a
webhook or endpoint at a private, loopback or link-local address (e.g.
127.0.0.1, 169.254.169.254 cloud metadata, RFC1918 hosts) and make the
server issue requests into its own internal network.

Add Postal::HTTP::AddressGuard, which resolves the destination host and
rejects private/loopback/link-local/reserved/multicast IPv4 and IPv6
addresses, then pins the connection to the validated address so it cannot
be redirected via a DNS-rebinding race. Administrators can permit specific
destinations via the new postal.allowed_request_destinations config option
(hostnames or IP/CIDR ranges).

Address selection only uses families this server can actually reach so we
do not pin to an IPv6 address on a host without IPv6 connectivity; IPv4 is
preferred for predictability. HTTPEndpoint now validates that its URL is a
well-formed HTTP(S) URL with a host.

spec/services/webhook_delivery_service_spec.rb

@@ -13,6 +13,7 @@ RSpec.describe WebhookDeliveryService do
   let(:response_body) { "OK" }
 
   before do
+    allow(Resolv).to receive(:getaddresses).with("example.com").and_return(["93.184.216.34"])
     stub_request(:post, webhook.url).to_return(status: response_status, body: response_body)
   end
 
@@ -116,5 +117,26 @@ RSpec.describe WebhookDeliveryService do
         expect { webhook_request.reload }.to raise_error(ActiveRecord::RecordNotFound)
       end
     end
+
+    context "when the webhook URL resolves to a blocked (private) address" do
+      let(:webhook_request) do
+        create(:webhook_request, :locked, webhook: webhook, url: "http://internal.example.com/hook")
+      end
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with("internal.example.com").and_return(["127.0.0.1"])
+      end
+
+      it "does not make a request to the destination" do
+        service.call
+        expect(WebMock).not_to have_requested(:post, "http://internal.example.com/hook")
+      end
+
+      it "records the failure and schedules a retry" do
+        service.call
+        expect(webhook_request.reload.attempts).to eq(1)
+        expect(webhook_request.retry_after).to be_present
+      end
+    end
   end
 end