Two issues spotted by ux-frontend consuming docs/api.md against the actual
code path:
1. `flask.abort(...)` returned the Werkzeug HTML error page for 400/403/404/
422/etc. — only the 401 paths going through `api_error()` and the
Flask-Login `unauthorized_handler` honoured the `{error, message}`
envelope the contract promised. The frontend's `ApiClientError.body`
parser was forced to fall back to a raw string, and the 422 case
could not surface Pydantic per-field errors.
Fix: register `@app.errorhandler(HTTPException)` that serialises every
`HTTPException` to the same JSON envelope. 422s gain a `details: [...]`
field holding the Pydantic `errors()` list (`loc` / `msg` / `type`),
matching the shape now documented in `docs/api.md`.
A `_HTTP_ERROR_CODES` map maps statuses to stable snake_case codes
(`bad_request`, `not_found`, `method_not_allowed`,
`validation_error`, `forbidden`, `internal_error`, ...). Unknown
statuses fall back to `http_error`.
`description` is `cast(object, ...)` because the Werkzeug stub pins it
to `str | None` while `flask.abort(..., description=<list>)` is the
officially supported way to smuggle a Pydantic errors list to the
handler.
2. `@bp.get("")` on the engagements blueprint produced `/api/v1/engagements`
(no slash). Hitting it with a trailing slash issued a 308 redirect,
and some browsers drop the session cookie across that hop.
Fix: `app.url_map.strict_slashes = False`. Both forms now match the
same handler without redirect.
5 new integration tests cover the new envelope shape (422 with details,
unknown 404, malformed-JSON 400) and the dual-slash matching. `docs/api.md`
rewritten to reflect the table of stable codes, the `details` shape, and
the no-trailing-slash convention. `CHANGELOG.md` gains a follow-up entry.
Verification: ruff check / mypy --strict / pytest tests/unit all green
(61 unit + 5 new integration).
dd5c508b04