fix(security): round 2 — the SEC-20 fix had four defects of its own

A second review round over the first round's fixes, which is where the remaining
real bugs usually are. All four are the same failure as the round it reviewed: a
claim stated in a comment that the code does not hold.

The middleware moved inside CORS, but the setup gate is registered LATER and is
therefore OUTSIDE it — so the comment's "the setup gate never runs for a body that
is already too large" was false the moment it was written. On an unconfigured
instance that gate opens a DB session per request, which is exactly the anonymous
pre-auth work SEC-20 exists to remove. The cap now sits between them, and the
order is asserted against `app.user_middleware` rather than described: prose about
a stack that `add_middleware` builds inside-out is how this got backwards.

The edge ceiling and the route's own cap were pinned EQUAL, but they measure
different things — the edge counts raw body bytes, the route counts the decoded
file. A file of exactly the advertised maximum was refused one layer before the
route that would have taken it. The edge now allows for the multipart envelope,
and the test asserts the relation and the gap instead of equality.

`sysconfig`'s new `max` was enforced on write only. That bound did not exist
before this epic, so a value that was legal when it was written can be sitting in
the database — served unclamped it produces exactly the drift the comment says is
impossible, with the admin page showing a number outside its own range. Int
overrides are clamped on read now, which also survives an env default being
lowered later.

And `DOWNLOAD_UPLOAD_MAX_BYTES=0` meant "no per-file cap" to the route but a hard
zero to the edge: every upload refused, including the admin page's own way back,
recoverable only by editing env and restarting. The setting takes `ge=1`.

Two test gaps from the same round: the header assertion that is the ONLY check of
the middleware's position was DB-gated and skipped on a host without Postgres, and
the headroom "ratio" the docblock calls load-bearing was held as `<=`, i.e. no
ratio at all. Both fixed, plus a sweep that every `capped_body` call site in the
routers is known and fits under its ceiling — the point of that list is the third
entry, not the two that exist today.

backend/app/config.py

@@ -1,7 +1,7 @@
 from pathlib import Path
 from typing import Literal, get_args, get_origin
 
-from pydantic import ValidationInfo, field_validator, model_validator
+from pydantic import Field, ValidationInfo, field_validator, model_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 # Shipped placeholder; production must override it. Used by the startup guard below.
@@ -247,7 +247,11 @@ class Settings(BaseSettings):
     # Per-file cap for local uploads (E7). Unlike a download (whose size is unknown until the worker
     # finishes), an upload's size is known at ingest, so this bounds a single file directly; the
     # user's overall byte footprint is still governed by download_default_max_bytes.
-    download_upload_max_bytes: int = 524_288_000  # 500 MiB
+    # ge=1: this number is also the EDGE ceiling (`bodylimit.ROUTE_MAX_BODY_BYTES`), where 0 is a
+    # real limit of zero rather than the route's "no per-file cap" sentinel — an instance set to 0
+    # would refuse every upload, including the admin page's own way back. To lift the cap in
+    # practice, set a large number.
+    download_upload_max_bytes: int = Field(default=524_288_000, ge=1)  # 500 MiB
     # Retention: an asset expires this many days after its last access; the GC job then
     # deletes it (once no job references it) after an additional grace window.
     download_retention_days: int = 30