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/quote.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module StrikeMe
+  module API
+    class Invoices
+      # Response of POST invoices/{id}/quote — the actual payable BOLT 11
+      # only exists once a quote is issued; Invoice#create alone reserves
+      # the invoice shell but has nothing a wallet can pay yet.
+      class Quote < BaseModel
+        attribute :quoteId
+        attribute :description
+        attribute :lnInvoice
+        attribute :onchainAddress
+        attribute :expiration, :datetime
+        attribute :expirationInSec, :integer
+        attribute :targetAmount
+        attribute :sourceAmount
+        attribute :conversionRate
+
+        alias_attribute :id, :quoteId
+        alias_attribute :bolt11, :lnInvoice
+        attribute_names.select { _1.end_with? 'Id' }.each do |snake_name|
+          alias_attribute snake_name.underscore, snake_name
+        end
+
+        def expired?
+          expiration.present? && expiration.past?
+        end
+      end
+    end
+  end
+end