Add invoice creation, quoting, and webhook signature verification

Finishes the two gaps that mattered most for real use: Invoices#create
(POST invoices) + Invoices#quote (POST invoices/{id}/quote, returning
the new Quote model with the actual payable bolt11 — an invoice alone
has nothing payable until quoted) and Webhook.valid_signature?
(HMAC-SHA256 over the raw body via X-Webhook-Signature, per Strike's own
documented reference implementation, timing-safe compare). Also finishes
Invoices#find (was a stub with a leftover binding.pry) and adds
WebhookEvent to parse the {id, eventType, data: {entityId, changes}}
payload shape — deliberately doesn't expose changed values, since
Strike's webhooks never carry them; a real value only ever comes from
re-fetching the resource.

Client gains #post/#patch (only #get existed before), needed for both
of the above.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0199wudpuAtkhouHKSEGxzpa

lib/strike_me/api/client.rb

@@ -26,6 +26,14 @@ module StrikeMe
       def get(...)
         @last_response = connection.get(...)
       end
+
+      def post(...)
+        @last_response = connection.post(...)
+      end
+
+      def patch(...)
+        @last_response = connection.patch(...)
+      end
     end
   end
 end