test(e2e): validate real responses against the OpenAPI schemas

[moveeeax]

The gates check only (method, path) — schema bodies had no witness. Now every JSON response the e2e suite already provokes is validated against the spec.

• tests/e2e/openapi_check.hpp (~300 lines, nlohmann-only — no new deps): the OpenAPI subset this repo actually uses (type incl. nullable arrays, required, properties, items, enum, $ref, bool additionalProperties); anything else is a logged SKIP, never a silent pass; an undocumented status is a failure.
• Spec → JSON at authoring time (scripts/gen-openapi-json.sh, committed openapi.gen.json) with an FNV-1a-64 freshness hash the test re-computes from the yaml — regeneration drift fails with instructions. Runtime YAML parsing rejected honestly: no pyyaml/node in the test image, and a C++ yaml dep rebuilds the vcpkg world.
• expect_matches_schema(...) wired onto every existing JSON call (17 call sites: auth flows incl. 415/422/401 branches, admin, posts, uploads, idempotency).

What it caught (all fixed on the spec side; the code told no lies):

1. Error/ValidationError schemas lacked the status field the server always sends.
2. The three pre-existing spectral errors on master — flow-map commas spawning phantom YAML keys — finally root-caused and fixed; make lint-openapi is 0-error for the first time.
3. Four real-but-undocumented statuses (login 415, register 422, jobs/uploads 401) documented.
4. POST /posts had no Post schema at all; Post, PostDetailResponse, RegisterResponse, healthz body added.

Gates + selftest 19/19 green; hash parity python↔C++ proven; the e2e run in this CI is the live proof.