fix(security): SEC-20's exemptions become raised ceilings, and the 413 is dressed

The module admitted its two opt-outs under a stated rule — "both enforce their own
limit on the bytes they actually read, which is what makes opting them out safe" —
and that rule is false for one of them. `POST /api/downloads/upload` declares
`file: UploadFile = File(...)`, so FastAPI parses and spools the whole multipart
part to a temp file BEFORE the first dependency runs, `require_human` included:
its in-handler cap measures bytes that have already landed. Measured on the
installed fastapi 0.141.1 / starlette 1.6.0 — starlette's max_part_size guards
non-file parts only, so an anonymous chunked upload was bounded by nothing. In the
shipped self-host compose that lands in a RAM-backed tmpfs.

The exposure predates SEC-20 (no route was capped before it); what this branch
added was the claim. So the list becomes a mapping of per-route CEILINGS instead
of a set of exemptions: every request now gets a finite number, the upload route's
being the same one the route itself enforces. `sysconfig`'s knob is bounded by it
so an admin cannot raise the handler's cap past the edge and start collecting 413s
from a layer the admin page never mentions — held by a test, not by prose.

Two more things the review caught in the same module:

The 413 was not byte-identical to `capped_body`'s after all. `json.dumps` puts
spaces where JSONResponse does not, and the test that promised to hold them
together compared PARSED JSON, so it could never have seen it. Built through the
same JSONResponse now, and compared as raw bytes.

And the middleware sat outside CORS and the security headers, so its 413 carried
neither — on the split-origin dev setup the browser drops it and the user gets a
generic network failure instead of the translated too_large, while the identical
refusal from capped_body one layer down renders fine. It moves to just inside
CORS: still ahead of the session, the setup gate and every auth dependency.

Also corrected: the docblock's "largest non-exempt body is 64 KiB, ~16x headroom"
(it is 512 KiB and 2x — the diagnostics sink, named eleven lines further down),
and CLAUDE.md's Request SIZE row, which forbade the one correct use of
capped_body that is actually in the tree.

CLAUDE.md

@@ -79,7 +79,7 @@ CQRS, a repository/DI framework, a router library, a generic CRUD-router factory
 | ------------------------------------ | ----------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------ |
 | ORM                                  | SQLAlchemy 2.0 `select()`                                                                                               | `db.query(...)`                                                                      |
 | Request body                         | a Pydantic model                                                                                                        | `payload: dict`                                                                      |
-| Request SIZE                         | nothing — `bodylimit` caps everything (SEC-20)                                                                          | `capped_body`, except for a LARGER per-route cap                                     |
+| Request SIZE                         | nothing — `bodylimit` caps every route (SEC-20); need more? add a ceiling to `ROUTE_MAX_BODY_BYTES`                      | `capped_body` on a route that does not read raw bytes itself                          |
 | Ownership check                      | `owned_or_404` / `get_or_404`                                                                                           | ad-hoc `_own_playlist`/`_owned`/`_own_job`                                           |
 | Admin gate import                    | `app.auth`                                                                                                              | the `app.routes.admin` re-export                                                     |
 | Upstream failure status              | `422` with JSON detail                                                                                                  | `502` (reserve 502 for raw byte-proxies)                                             |