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/webhook_event.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module StrikeMe
+  module API
+    # Parses the WebhookEvent payload every Strike webhook POST carries —
+    # {id, eventType, webhookVersion, data: {entityId, changes}, created,
+    # deliverySuccess}. Deliberately does NOT carry the changed values
+    # themselves — Strike's own docs are explicit that the event never
+    # contains the actual data changes, only that something changed on
+    # `entity_id`. A real value only comes from calling the matching
+    # fetch endpoint (e.g. Invoices#find for an invoice.* event) —
+    # trusting anything else here would mean trusting an unauthenticated
+    # claim about money having moved.
+    class WebhookEvent < BaseModel
+      attribute :id
+      attribute :eventType
+      attribute :webhookVersion
+      attribute :data
+      attribute :created, :datetime
+      attribute :deliverySuccess, :boolean
+
+      alias_attribute :event_type, :eventType
+      alias_attribute :webhook_version, :webhookVersion
+      alias_attribute :delivery_success, :deliverySuccess
+
+      def entity_id
+        data['entityId'] || data[:entityId]
+      end
+
+      def changes
+        data['changes'] || data[:changes] || []
+      end
+
+      def invoice_event?
+        event_type.to_s.start_with?('invoice.')
+      end
+    end
+  end
+end