New: Add POST /validate endpoint to AbstractApiModule

[taylortom]

Fixes #110

New

• Add a POST /validate route to default-routes.json, inherited by every AbstractApiModule subclass. modifying: false so the raw body reaches the handler un-sanitised (we validate exactly what was sent).
• Add validateHandler() to AbstractApiModule: validates req.apiData.data against the resolved schema (honouring subclass getSchemaName() overrides), responds 200 with the validated/normalised document, or forwards the schema validation error via next().
• Gated on write:${scope} — validation is a pre-write operation; read-only users have no create/edit flow and so no use for it.

Docs

• Document the new inherited route in both endpoint tables in writing-an-api.md.

Testing

• #validateHandler(): responds 200 with the validated data (asserts schema name + body + options passed through); forwards a validation error to next and sends no response.
• default-routes.json: /validate present, modifying: false, gated on the write scope.
• Full suite (66 tests) passes; npx standard clean.

[github-actions]

🎉 This PR is included in version 3.10.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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