fix(webhook): create a Forgejo-native hook so /forgejo setup webhook works

[hstern]

Problem

/forgejo setup webhook (and the setup wizard's webhook step) always fails against a Forgejo server with:

[Type]: Required

submitWebhook (server/plugin/flows.go) builds a *github.Hook{} and calls the go-github client.Repositories.CreateHook / Organizations.CreateHook. Forgejo's create-hook API doesn't accept GitHub's hook shape — it uses its own CreateHookOption, where type is a required field. go-github never sends type, so Forgejo rejects every create. The webhook can never be created, and since a channel can't be subscribed without a webhook, the notification path is unreachable.

There's a second, latent issue right after: the code blocks on pingBroker.SubscribePings() waiting for a ping event matching the new hook ID. Forgejo doesn't emit a ping on hook creation (unlike GitHub), so even if create succeeded that loop would only ever hit the 28s timeout and report failure.

Fix

• Build Forgejo's native payload (type: "forgejo", config.content_type: "json", config.secret = the plugin's own WebhookSecret) and POST it to {baseURL}api/v1/repos/{owner}/{repo}/hooks (or …/orgs/{org}/hooks) using the user's already-authenticated client (client.Client()). Because the hook is created with the plugin's WebhookSecret, incoming deliveries verify against the same secret (X-Hub-Signature), and the plugin already reads the X-Forgejo-Event header that type: forgejo hooks send.
• Drop the post-create ping wait — a successful create is the confirmation.
• Use Forgejo's event names for the subscription (pull_request_comment, pull_request_review, …).

Status / testing

• go build ./... passes; the change is confined to submitWebhook.
• The [Type]: Required failure was reproduced live (Forgejo + Mattermost Team 11.7.2): the existing /forgejo setup webhook dialog returns {"error":"[Type]: Required"}, and a manually-created type: forgejo hook is accepted by the same Forgejo API, confirming the missing-type diagnosis.
• End-to-end verification of the rebuilt plugin (create succeeds → subscribed channel receives events) is in progress; I'll follow up here with the result.

Happy to adjust the hook type (forgejo vs gitea), event list, or add tests per your preference.

🤖 Generated with Claude Code

[hstern]

End-to-end verified on a live Forgejo + Mattermost Team 11.7.2 install with the rebuilt plugin:

• /forgejo setup webhook → entered a repo → succeeds (no more [Type]: Required).
• Forgejo now shows the created hook: type: forgejo, active, URL …/plugins/forgejo/webhook.
• Subsequent issue/PR/push events on that repo are delivered, signature-verified by the plugin, and posted into the subscribed Mattermost channel.

So both the create fix and the dropped ping-wait check out in practice. Happy to tweak the hook type/event list or add a test if you'd like.