fix(uploads): read the body after authenticating, not before it

`POST /api/downloads/upload` declared `file: UploadFile = File(...)`, and
FastAPI parses and spools a declared body before the first dependency runs --
`require_human` included. An unauthenticated caller could therefore make the
process write the whole per-file allowance (500 MiB by default) to temp, once
per request. SEC-20's ceiling bounded that number but could not stop the
writing, which is why E27 parked it rather than closing it.

The route now takes no body parameter. `spool_upload` names `require_human`
first and only then streams `request.stream()` to the staging temp file, so a
stranger is refused before a byte moves -- the shape the screenshot route has
used since E16. Filename and display name travel as percent-encoded headers
(latin-1 on the wire, so a Hungarian filename needs the encoding) instead of
multipart parts.

`capped_stream_to` joins `capped_body` in routes/_common.py: the same counting
rule, written to a file descriptor rather than accumulated, because holding
500 MiB would trade a disk problem for a memory one. Its refusal is the
caller's, so the upload route keeps answering 422 `upload_too_large` with the
admin knob's own number.

With no envelope to allow for, the edge is now exactly that knob, and
MULTIPART_ENVELOPE_ALLOWANCE is gone.

The invariant stops being prose: test_body_limit.py asks FastAPI itself
(`route.dependant.body_params`) and fails the gate if any route with a raised
ceiling declares a body. A comment could not have prevented the defect it
described; that test would have.

Both new tests were seen red for the right reason -- before the fix the
anonymous probe got its 401 only after the server had read 4236 bytes of a
multipart body, and re-declaring the File parameter turns the invariant test
red naming the route and the parameter.

backend/tests/test_body_limit.py

@@ -251,14 +251,57 @@ def test_the_upload_ceiling_and_the_admin_knob_cannot_drift_apart():
     constant. If the knob could be set above the edge, an admin raising it would silently start
     getting 413s from a layer the admin page never mentions.
 
-    The relation is `<=`, not `==`, and the gap is the point: the edge counts RAW body bytes while
-    the route counts the DECODED file, so the edge has to allow for the multipart envelope or a file
-    of exactly the advertised maximum is refused one layer before the route that would take it.
+    Since P1 the two count the SAME bytes — the body is the file, with no multipart envelope
+    wrapped round it — so the old allowance is gone and the edge is the knob's own maximum,
+    exactly. A gap reappearing means someone re-wrapped the upload in an envelope without saying so.
     """
     spec = next(s for s in sysconfig.SPECS if s.key == "download_upload_max_bytes")
     edge = bodylimit.ROUTE_MAX_BODY_BYTES[("POST", "/api/downloads/upload")]
-    assert spec.max is not None and spec.max <= edge
-    assert edge - spec.max == bodylimit.MULTIPART_ENVELOPE_ALLOWANCE
+    assert spec.max == edge
+
+
+def _route_objects(root):
+    """`_declared_routes`' sibling, keeping the route OBJECT so its FastAPI dependant can be read."""
+    found: dict[tuple[str, str], object] = {}
+    stack = [root]
+    while stack:
+        node = stack.pop()
+        for route in getattr(node, "routes", ()) or ():
+            inner = getattr(route, "original_router", None)
+            if inner is not None:
+                stack.append(inner)
+                continue
+            for method in getattr(route, "methods", None) or ():
+                found[(method, route.path)] = route
+    return found
+
+
+@pytest.mark.parametrize(("method", "template"), sorted(bodylimit.ROUTE_MAX_BODY_BYTES))
+def test_no_route_with_a_raised_ceiling_declares_a_body_parameter(method, template):
+    """P1's invariant, and the reason this is a test rather than a comment.
+
+    A raised ceiling is only honest when the route's own cap applies to the bytes AS THEY ARRIVE.
+    FastAPI reads and spools a DECLARED body (`= File(...)`, `= Form(...)`, a Pydantic model)
+    BEFORE the first dependency resolves — so on such a route this middleware's ceiling is the only
+    thing between an anonymous caller and the whole allowance, `require_human` never gets a say, and
+    the route's in-handler cap measures bytes that have already landed. That is exactly what P1
+    closed on the upload route, which sat in `ROUTE_MAX_BODY_BYTES` with a 500 MiB ceiling and a
+    `file: UploadFile = File(...)` body for one release.
+
+    Asked of FastAPI itself (`route.dependant.body_params`) rather than parsed or grepped: the
+    router already knows which parameters it will read the body for, and a shape a regex or an AST
+    walk guesses wrong is precisely how the next route slips through."""
+    from app.main import app
+
+    route = _route_objects(app).get((method, template))
+    assert route is not None, f"{method} {template} is not a route any more"
+    body_params = [p.name for p in getattr(route.dependant, "body_params", ())]
+    assert not body_params, (
+        f"{method} {template} declares a body parameter ({', '.join(body_params)}), so FastAPI "
+        "spools it before the first dependency runs and the raised ceiling is all that stands "
+        "between an anonymous caller and the whole allowance. Read the body yourself, from a "
+        "dependency declared AFTER the auth one — see routes/downloads.py:spool_upload."
+    )
 
 
 def test_a_stored_override_outside_the_bounds_falls_back_on_READ(db):