Fix critical webhook signature case bug; add correlationId + AR type registration

Strike sends X-Webhook-Signature in UPPERCASE hex. OpenSSL::HMAC.
hexdigest always returns lowercase, and valid_signature? compared them
case-sensitively via secure_compare — every genuine Strike webhook was
being silently rejected as an invalid signature. Confirmed live: executed
a real Lightning payment (payment-quotes/lightning + PATCH .../execute),
received a real payment.updated webhook, and recomputing the HMAC over
the captured payload/secret matched case-insensitively but not
case-sensitively — proving the algorithm/secret/body were already
correct and this was purely a case-comparison bug. Their own reference
docs (docs.strike.me/webhooks/signature-verification) don't mention the
case difference. Regression test added using the real captured signature,
upcased.

Also, both found via that same live test:
- Invoices::Invoice didn't declare correlationId, raising
  ActiveModel::UnknownAttributeError the moment a real one (which
  CreateInvoice always sends) round-tripped through a live response —
  no existing spec fixture's stubbed body had included it.
- BtcAttributeType was only registered with ActiveModel::Type, not
  ActiveRecord::Type (a separate registry) — an `attribute :x,
  :strike_btc` declaration on an ActiveRecord model raised "Unknown
  type" without this. Guarded behind defined?(ActiveRecord::Type) since
  this gem only depends on active_model.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

spec/strike_me/api/invoices_spec.rb

@@ -13,27 +13,29 @@ RSpec.describe StrikeMe::API::Invoices do
         expect(list.last ).not_to be_paid
 
         expect(list.first.attributes).to eq(
-          'invoiceId'   => '97dca64c-6dde-4a87-b970-279f1a8e0ac5',
-          'amount'      => {'amount' => '0.00014', 'currency' => 'BTC'},
-          'state'       => 'PAID',
-          'created'     => '2025-08-08 22:28:44.195825 +0000', # is actual Time
-          'description' => 'Hi there',
-          'issuerId'    => '41952e07-483c-4cd7-afa4-b6a8ec729809',
-          'receiverId'  => '41952e07-483c-4cd7-afa4-b6a8ec729809',
-          'payerId'     => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c'
+          'invoiceId'     => '97dca64c-6dde-4a87-b970-279f1a8e0ac5',
+          'amount'        => {'amount' => '0.00014', 'currency' => 'BTC'},
+          'state'         => 'PAID',
+          'created'       => '2025-08-08 22:28:44.195825 +0000', # is actual Time
+          'description'   => 'Hi there',
+          'issuerId'      => '41952e07-483c-4cd7-afa4-b6a8ec729809',
+          'receiverId'    => '41952e07-483c-4cd7-afa4-b6a8ec729809',
+          'payerId'       => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c',
+          'correlationId' => nil
         )
         expect(list.first.id     ).to eq '97dca64c-6dde-4a87-b970-279f1a8e0ac5'
         expect(list.first.created).to be_a Time
 
         expect(list.last.attributes).to eq(
-          'invoiceId'   => 'f3853f94-d198-4b83-8b9a-11a836926819',
-          'amount'      => {'amount' => '40.00', 'currency' => 'USDT'},
-          'state'       => 'UNPAID',
-          'created'     => '2023-05-20 22:44:36.856775 +0000',
-          'description' => '',
-          'issuerId'    => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c',
-          'receiverId'  => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c',
-          'payerId'     => nil
+          'invoiceId'     => 'f3853f94-d198-4b83-8b9a-11a836926819',
+          'amount'        => {'amount' => '40.00', 'currency' => 'USDT'},
+          'state'         => 'UNPAID',
+          'created'       => '2023-05-20 22:44:36.856775 +0000',
+          'description'   => '',
+          'issuerId'      => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c',
+          'receiverId'    => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c',
+          'correlationId' => nil,
+          'payerId'       => nil
         )
       end
     end

spec/strike_me/api/webhook_spec.rb

@@ -10,6 +10,17 @@ RSpec.describe StrikeMe::API::Webhook do
       expect(described_class.valid_signature?(raw_body: raw_body, signature: real_signature, secret: secret)).to eq true
     end
 
+    # Strike sends X-Webhook-Signature in UPPERCASE hex — confirmed live
+    # against a real payment.updated delivery 2026-09-22 (executed a real
+    # Lightning send, received a real signed webhook). OpenSSL::HMAC.
+    # hexdigest always returns lowercase; without downcasing both sides
+    # this rejected every genuine Strike webhook, silently, since a
+    # rejected webhook is indistinguishable from an actual forgery
+    # attempt from outside.
+    it 'is true for the real signature even when Strike sends it as UPPERCASE hex' do
+      expect(described_class.valid_signature?(raw_body: raw_body, signature: real_signature.upcase, secret: secret)).to eq true
+    end
+
     it 'is false for the wrong secret' do
       expect(described_class.valid_signature?(raw_body: raw_body, signature: real_signature, secret: 'wrong')).to eq false
     end