spp_api_v2: error responses are FastAPI {detail}, not the RFC 9457 Problem Details the API principles specify #556

Problem

OpenSPP's API guidelines specify RFC 9457 Problem Details (type, title, status, detail,
application/problem+json), and spp_api_v2/schemas/problem_detail.py defines a ProblemDetail schema. But no
spp_api_v2* module registers an exception handler: routers raise HTTPException, and the OCA fastapi error path
(fastapi/error_handlers.py, convert_exception_to_status_body) renders {"detail": ...} as application/json.
The principle doc already carries a note that "a global exception handler should be added".

Examples: 404 {"detail": "Individual not found"}, 422 {"detail": [{"loc": [...], "msg": ...}]}.

Clients (e.g. the OpenFn language-openspp v4 adaptor) currently have to parse both shapes. #434 (spp_attendance
alignment) assumes the v2 stack already emits RFC 9457.

Proposal

Render ProblemDetail (application/problem+json, URN type per the principle's category table) for all
spp_api_v2 endpoints, including bundle per-entry errors and 422 validation errors (errors[]). This changes every
error body, so it needs a release note and ideally a deprecation window (e.g. keep detail populated, which RFC 9457
allows).

Context

Item C of #554 (deferred from PR #555 as a contract change).