Generate webhook signing secrets server-side and let users reveal them on
demand, replacing the create-dialog secret controls that surfaced internal
mask state (Status / Generate & Copy / Clear / Pending) to users.
- Add owner-gated GetUserWebhookSigningSecret RPC — the only path that
returns the secret; list/create/update responses still omit it.
- Generate the secret server-side on create (webhook.GenerateSigningSecret),
so validity no longer depends on the client.
- Rename UserWebhook.has_signing_secret -> signing_secret_set for parity
with the existing api_key_set field.
- Create dialog drops the secret section to a one-line note; the generated
secret is shown once right after create and revealable from Edit later.
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