feat(security): cap every request body before auth runs (SEC-20)

FastAPI reads a declared body IN FULL before the first dependency, so
`POST /auth/password-login` drained an arbitrary upload from an anonymous
caller and only then answered 401. 70 of the 72 body-declaring routes
behaved the same way: one unauthenticated client could spend the
process's memory and time at will.

A raw ASGI middleware, not `@app.middleware("http")` — BaseHTTPMiddleware
cannot wrap `receive`, so it could only trust `Content-Length`, which a
client may omit (chunked) or understate. This refuses an oversized
declaration outright and counts the bytes of everything else as they
arrive, the same rule `capped_body` already applies per-route. Registered
directly inside the access log: a refusal is still logged with its
request id, but the session, CORS, the setup gate and every auth
dependency never run for a body that is already too large.

Two routes opt out, under one stated rule: an exemption is ONLY for a
route whose own cap is larger than the global one. That is why
`/api/diagnostics/events` is absent — it caps itself at 512 KiB, below
the ceiling, so exempting it could only widen what it accepts. The list
names route templates and a test resolves each against the live route
table, so a rename fails the gate instead of silently dropping an
exemption in prod.

The 413 is byte-identical to `capped_body`'s and the test compares the
two against each other rather than against a literal — two shapes for one
fact is how the `cancelled`/`canceled` bug happened.

CLAUDE.md

@@ -57,7 +57,7 @@ how it gets built. Concretely:
 
 `routes/*` (HTTP only: validation, auth-dependency, serialization) → domain packages `sync/`, `plex/`,
 `downloads/`, `youtube/`, `issues/` (shared by routes + scheduler + worker) → flat core (`auth`, `quota`,
-`sysconfig`, `state`, `audit`, `notifications`, `userscope`, `realtime`, `ratelimit`, `progress`,
+`sysconfig`, `state`, `audit`, `notifications`, `userscope`, `realtime`, `ratelimit`, `bodylimit`, `progress`,
 `security`, `logs`, `logsink`, `diagnostics`, `debugsession`) → `models/db/config`. **Dependency direction is one-way: core never imports routes.**
 Frontend: `main.tsx` provider tree → `App.tsx` shell → `moduleRegistry` route-chunks → module
 components → the **platform layer `components/ui/`** (domain-free primitives: the popover engine and
@@ -78,6 +78,7 @@ 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       |
 | 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)               |