feat(#528): validate branding and partners configuration before upload

[AchrafReyani]

Description

upload-branding and upload-partners currently accept whatever is in branding.json / partners.json and upload it verbatim as the branding / partners doc. cht-core assumes a specific structure for these docs (api/src/services/branding.js, the admin images-branding / images-partners controllers): a string title, and a resources object whose values are attachment names. A malformed doc breaks the admin images pages without any indication of the cause (see the issue).

This PR validates both configuration files before anything is uploaded:

• Schema validation (joi, same approach as validate-app-settings.js):
  • branding.json: title is a required non-empty string; resources (optional) is an object whose logo / favicon / icon values are non-empty strings — the three keys cht-core reads.
  • partners.json: resources is a required object mapping partner name → non-empty string.
• Attachment check: every resources value must reference a file that exists in the branding/ or partners/ directory, otherwise cht-core would look up an attachment that was never uploaded.

On failure the action throws a descriptive error and nothing is written, e.g.

Invalid branding configuration in /project/branding.json: "title" must be a string; "resources" must be of type object
Invalid branding configuration in /project/branding.json: "resources.favicon" references "fav.ico" but no such file was found
Invalid partners configuration in /project/partners.json: "resources" must be of type object

Implementation: new src/lib/validate-configuration-docs.js; upload-configuration-docs takes an optional validate(settings, attachments) callback (alongside the existing optional processJson), and the two actions pass their validator. Other callers of upload-configuration-docs are unaffected.

#528

Code review items

• Readable: Concise, well named, follows the style guide, documented if necessary.
• Documented: The accepted structure is already what cht-docs describes; happy to add a note about the new validation errors there if you'd like.
• Tested: 17 new unit tests in test/lib/validate-configuration-docs.spec.js plus new cases in test/lib/upload-configuration-docs.spec.js (validate accepted / rejected → throws, no upload); action specs assert the validator is wired in.

Verification

• Reproduced on main with the upload module stubbed: { "title": 42, "resources": "logo.png" } is uploaded as-is. After this change the same input throws the first error above and insertOrReplace is never called; a valid config still uploads the same doc as before.
• npm run eslint clean.
• npm test in a clean node:22 container with FORCE_COLOR=1 (same as CI): 899 passing, 1 pending (the pending one is pre-existing), 0 failing.

Written with assistance from Claude Code; every change and test result above was run and checked locally.