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

strike_me-api.gemspec

@@ -8,15 +8,14 @@ Gem::Specification.new do |spec|
   spec.authors = ['Benjamin ter Kuile']
   spec.email = ['bterkuile@gmail.com']
 
-  spec.summary = 'TODO: Write a short summary, because RubyGems requires one.'
-  spec.description = 'TODO: Write a longer description or delete this line.'
-  spec.homepage = 'TODO: Put your gem website or public repo URL here.'
+  spec.summary = 'Ruby client for the Strike (strike.me) API'
+  spec.description = 'Invoice creation/quoting, account/balance reads, and webhook signature verification for the Strike Lightning payments API.'
+  spec.homepage = 'https://gitlab.com/benja-2/strike_me-api'
   spec.license = 'MIT'
   spec.required_ruby_version = '>= 3.1.0'
 
   spec.metadata['homepage_uri'] = spec.homepage
   spec.metadata['source_code_uri'] = 'https://gitlab.com/benja-2/strike_me-api'
-  spec.metadata['changelog_uri'] = 'TODO: Put your gem CHANGELOG.md URL here.'
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.