offers above/below Footer for clients

[Rekoo-PS]

The idea is to make a custom space for offers, like : banner/text/advertisement .

can be made /per event
or globally.

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example :
i wanna give this client large prints offer > i'll drop it in his event .
if not, then the global offer will appear.

[the-luap]

Implemented in #450 (targeting beta).

Format: Markdown only (no raw HTML). Source goes through marked → DOMPurify with a tight allowlist (no <img>, no tables, no <script>/<style>), and every link gets target="_blank" rel="noopener noreferrer nofollow" after sanitize.

Global default: Branding settings → "Gallery Promotional Banner" — markdown content + position selector (above footer | below footer). Live preview included.

Per-event override: Edit Event form now has a three-way switch:

• inherit (default) — uses the global content as-is.
• custom — overrides with this event's own markdown, hidden from other galleries.
• off — suppresses the slot entirely for that event, even if the global one is set.

Backend nulls promo_markdown automatically when mode != custom so stale override text never persists after a mode switch.

Bundled with the footer changes from #441 since they share the same component surface.

[Luca-Timo]

Long open Issue, I will close now, just reopen the issue if the bug is no solved