feat: Linear OAuth 2.0 (#10851)

Fixes https://linear.app/chatwoot/issue/CW-3417/oauth-20-authentication
We are planning to publish the Chatwoot app in the Linear [integration
list](https://linear.app/docs/integration-directory). While we currently
use token-based authentication, Linear recommends OAuth2 authentication.
This PR implements OAuth2 support.

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

app/helpers/linear/integration_helper.rb

@@ -0,0 +1,47 @@
+module Linear::IntegrationHelper
+  # Generates a signed JWT token for Linear integration
+  #
+  # @param account_id [Integer] The account ID to encode in the token
+  # @return [String, nil] The encoded JWT token or nil if client secret is missing
+  def generate_linear_token(account_id)
+    return if client_secret.blank?
+
+    JWT.encode(token_payload(account_id), client_secret, 'HS256')
+  rescue StandardError => e
+    Rails.logger.error("Failed to generate Linear token: #{e.message}")
+    nil
+  end
+
+  def token_payload(account_id)
+    {
+      sub: account_id,
+      iat: Time.current.to_i
+    }
+  end
+
+  # Verifies and decodes a Linear JWT token
+  #
+  # @param token [String] The JWT token to verify
+  # @return [Integer, nil] The account ID from the token or nil if invalid
+  def verify_linear_token(token)
+    return if token.blank? || client_secret.blank?
+
+    decode_token(token, client_secret)
+  end
+
+  private
+
+  def client_secret
+    @client_secret ||= ENV.fetch('LINEAR_CLIENT_SECRET', nil)
+  end
+
+  def decode_token(token, secret)
+    JWT.decode(token, secret, true, {
+                 algorithm: 'HS256',
+                 verify_expiration: true
+               }).first['sub']
+  rescue StandardError => e
+    Rails.logger.error("Unexpected error verifying Linear token: #{e.message}")
+    nil
+  end
+end