fix(db-postgres): find custom id field from flattened fields for FK type

[alphanull]

What?

When a collection defines a custom text or number id field nested inside a layout container (row, group, collapsible, or tabs), traverseFields in @payloadcms/drizzle used a shallow Array.find on collection.fields to determine the FK column type for relationship fields.

Layout containers have no name property, so fieldAffectsData returns false for them — the nested id field is never reached, and FK columns default to integer. This causes a hard startup crash when the referenced collection uses a varchar primary key:

detail: 'Key columns "header_id" and "id" are of incompatible types: integer and character varying.'
code: '42804'

Why?

traverseFields in the Drizzle adapter needs to find the custom id field of a related collection to determine the correct FK column type. The shallow find works only when id is at the top level of fields. Any layout wrapper breaks detection.

How?

Use flattenAllFields (already exported from payload/shared) before searching for the custom id field — identical to the fix applied to db-mongodb in PR #15110 (v3.71.0). This covers all layout containers: row, group, collapsible, and tabs.

Checklist

• Tests added (test/relationships — new customIdRowSlug collection with id nested in type: 'row', regression test verifies create/read works without FK type mismatch)
• Existing tests pass (59 passed, 0 failed against Postgres)
• No new dependencies

Closes #13695