dxa-gallery exporter: each partner’s own project, for legacy’s badge (#2476)

dxa-gallery exporter: each partner's own project, for legacy's badge (#2476)

* dxa-gallery exporter: each partner's own project, for legacy's badge

Legacy's gallery Partners page badges every partner with its project and
its tier there ("Discover Islamic Art Partner", "Sharing History
Affiliate"). partners.json now carries `own_project`: the project's id,
name and legacy key, the partner's level in that project's collection,
and whether the project is an online exhibition's. Checked on Ceramics:
the badge it gives matches legacy's on all 106 partners.

Part of #2423

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* dxa-gallery exporter: say why MWNF-371 isn't reproduced

Legacy's not-live-project filter is not a no-op: it hides The Hijaz
Railway's partners from the other galleries until that exhibition's live
date. A package built once cannot follow a date; it lists them, as legacy
does once the exhibition is live.

Part of #2423

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>