feat: repo-level extra metadata settings in UI, merged into release metadata

[slint]

Is your feature request related to a problem? Please describe.

Handling release metadata is a pain point at the moment. We currently extract it from the release objects of each VCS integration, and this has been customized historically in various ways:

• A .zenodo.json checked in the repository, containing metadata defaults or overrides
• The more standard CITATION.cff, whose fields can be mapped to record metadata. The issue though, is that not all record metadata fields have an equivalent in CITATION.cff.

Describe the solution you'd like

Add a repo-level "extra metadata" settings JSON field, configurable via the VCS UI.
This should allow users to define partial metadata,
that will be taken into account as part of the release.

At release handling time, we merge metadata (if present) in a clear and deterministic order:

1. release event object metadata
2. .zenodo.json
3. CITATION.cff mapping
4. repo settings "extra metadata"

Bonus

Having an example "preview" of the produced metadata would be fantastic (taking into account CITATION.cff as well).

Additional context

Zenodo docs currently describe support for both CITATION.cff and .zenodo.json,
with .zenodo.json taking precedence when both are present. This is done because CITATION.cff doesn't support all the metadata fields (e.g. communities, funding, ...)
Adding a VCS metadata settings layer would give a practical migration path away from .zenodo.json and promote a cleaner CITATION.cff-first workflow.