Optional "Legal notice" (Impressum) link next to Privacy and Terms

[moritzschieder]

Hi, and thanks for Calnode. While self-hosting it (v0.10.1), I ran into a legal gap on the public pages.

Problem

In Germany and Austria, a business website must show a legal notice ("Impressum") that is easy to find and clearly labelled (Germany: § 5 DDG, formerly § 5 TMG; Austria: § 5 ECG and § 25 MedienG). A public booking page counts as part of that web presence.

Branding has privacy_url and terms_url, but no field for a legal notice. The only workaround is to put the Impressum URL into terms_url. The footer then labels it with the terms key, which is "AGB" in de.json (general terms and conditions). So the Impressum shows up under the wrong label, which defeats the "clearly labelled" requirement. The translation itself is correct; the field is missing.

Affected: the footer in templates/_shared.html, used by the booking, manage and directory pages.

Proposal

• A new optional branding field legal_notice_url (PATCH /v1/settings/branding, plus the admin UI).
• If set, the footer renders a third link with a new i18n key legal_notice: "Impressum" (de), "Legal notice" (en), "Mentions légales" (fr). The other locales would follow your usual machine-draft process.
• If unset, nothing changes. It is purely additive, with no migration impact beyond one nullable column.

If this direction works for you, I am happy to send a PR: migration, handler, template, locale keys and a test. I would sign the CLA.