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>

lib/strike_me/api/invoices/invoice.rb

@@ -12,6 +12,15 @@ module StrikeMe
         attribute :issuerId
         attribute :receiverId
         attribute :payerId
+        # Only present on the #create response, echoing back whatever
+        # Invoices#create sent as correlationId (Strike's own docs list
+        # it as request-optional/response-present) — missing here meant
+        # ActiveModel::UnknownAttributeError the first time a real
+        # correlation_id round-tripped through a live Strike account (no
+        # existing spec fixture's stubbed response body included it, so
+        # this went uncaught until now). end_with?('Id') below already
+        # picks this up automatically, same as every other *Id field.
+        attribute :correlationId
 
         alias_attribute :id, :invoiceId
         attribute_names.select { _1.end_with? 'Id' }.each do |snake_name|

lib/strike_me/api/webhook.rb

@@ -22,7 +22,18 @@ module StrikeMe
         return false if signature.blank? || raw_body.blank?
 
         expected = OpenSSL::HMAC.hexdigest('SHA256', secret, raw_body)
-        secure_compare(expected, signature)
+        # Strike sends X-Webhook-Signature in UPPERCASE hex — confirmed
+        # live against a real webhook delivery (payment.updated, a real
+        # executed Lightning send) 2026-09-22. OpenSSL::HMAC.hexdigest
+        # always returns lowercase. Without downcasing both sides here,
+        # secure_compare's case-sensitive byte comparison rejected every
+        # genuine Strike webhook as an invalid signature — silently: a
+        # rejected webhook looks identical to an actual forgery attempt
+        # from the outside, so this would have broken settlement in
+        # production with no visible error at all. Their own docs'
+        # reference implementation (docs.strike.me/webhooks/
+        # signature-verification) doesn't mention this case difference.
+        secure_compare(expected.downcase, signature.downcase)
       end
 
       # ActiveSupport::SecurityUtils.secure_compare raises on a length

lib/strike_me/btc_attribute_type.rb

@@ -17,3 +17,12 @@ module StrikeMe
 end
 
 ActiveModel::Type.register(:strike_btc, StrikeMe::BtcAttributeType)
+# ActiveRecord keeps its own type registry (ActiveRecord::Type::
+# AdapterSpecificRegistry), separate from ActiveModel::Type.registry
+# above — an `attribute :amount, :strike_btc` declaration on an
+# ActiveRecord model looks it up there, not here, and raises "Unknown
+# type :strike_btc" without this second registration (confirmed live).
+# Guarded: this gem itself only depends on active_model, not
+# active_record — a plain ActiveModel::Model consumer never loads
+# ActiveRecord at all, and this must not be what breaks that.
+ActiveRecord::Type.register(:strike_btc, StrikeMe::BtcAttributeType) if defined?(ActiveRecord::Type)

spec/strike_me/api/invoices_spec.rb

@@ -20,7 +20,8 @@ RSpec.describe StrikeMe::API::Invoices do
           'description'   => 'Hi there',
           'issuerId'      => '41952e07-483c-4cd7-afa4-b6a8ec729809',
           'receiverId'    => '41952e07-483c-4cd7-afa4-b6a8ec729809',
-          'payerId'     => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c'
+          '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
@@ -33,6 +34,7 @@ RSpec.describe StrikeMe::API::Invoices do
           'description'   => '',
           'issuerId'      => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c',
           'receiverId'    => 'f75c22bf-625f-4c47-acdf-acb9c596bb8c',
+          'correlationId' => nil,
           'payerId'       => nil
         )
       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