[Feat] Add customizable account-linking help

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• added an admin-only Account linking help editor under Settings > Users
• appended the optional deployment guidance to source-control, Discord, and Telegram account-link prompts while preserving built-in provider instructions
• displayed the same guidance below “Ask your admin” on the unauthenticated sign-in form, including Markdown CTA links
• kept Slack, Teams, short Discord DM acknowledgements, and Telegram callback toasts unchanged
• documented the setting and added coverage for persistence, UI behavior, provider messages, and fail-open fallback behavior

Why this change was made

Community and self-hosted deployments may require users to request an invite before they can link an account. Admins can now add deployment-specific contact or invite instructions without replacing Roomote's maintained provider-aware copy.

Impact

Unlinked users receive the deployment's extra guidance at the point where they need it across source-control comments, Discord, Telegram, and the sign-in page. Blank settings preserve the current messages and sign-in layout, and a settings lookup failure falls back to the built-in copy rather than blocking integration responses.

Screenshots

[roomote-community]

1 issue outstanding. See task

• apps/web/src/app/(unauthenticated)/sign-in/[[...sign-in]]/page.tsx:51 Handle a failed account-link-help settings lookup. Calling getDeploymentAccountLinkHelpText() directly makes the whole sign-in page fail during a database outage, unlike integration prompts which deliberately fall back to built-in text. Catch the lookup failure here and pass null so existing users can still sign in.

_{Reviewed e9f2750}