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/invoices/invoice.rb

@@ -13,6 +13,7 @@ module StrikeMe
         attribute :receiverId
         attribute :payerId
 
+        alias_attribute :id, :invoiceId
         attribute_names.select { _1.end_with? 'Id' }.each do |snake_name|
           alias_attribute snake_name.underscore, snake_name
         end
@@ -20,6 +21,18 @@ module StrikeMe
         def paid?
           state == 'PAID'
         end
+
+        def unpaid?
+          state == 'UNPAID'
+        end
+
+        def pending?
+          state == 'PENDING'
+        end
+
+        def cancelled?
+          state == 'CANCELLED'
+        end
       end
     end
   end