Connect a channel by issuing a one-time code the user posts as /connect <code>
in their channel. A secret-token-guarded webhook matches the code, links the
channel as a SocialAccount (chat_id in meta), and records it on the request so
the connect endpoint can poll for completion. Adds the TelegramConnectRequest
model + migration, the connect/status endpoints, the public webhook route (CSRF
exempt), a ConnectionVerifier branch (getChat liveness), and a telegram:set-webhook
command. Tests cover the code issue, webhook link, secret rejection, expired/
unknown codes, status polling, and the command.
