Two limits governed a matrix upload and nothing kept them in
agreement: the documented option reached the homeserver, while the
nginx location every client request traverses carried its own
`client_max_body_size 50M`. Raising the option past 50M changed
nothing — nginx returned 413 at the proxy, naming neither matrix nor
the option that had just been raised.
The cap is not set equal to the option. Equality would regress the
default: at 20 MB a 25 MB upload is currently rejected by the
homeserver, with a matrix error a client can act on, and equality
turns that into a gateway 413. The proxy must never be the party that
rejects, so it tracks the option with headroom. This is the shape the
forge vhost already uses for git, where a generous proxy cap lets the
application own the real limit.
The module-eval arm sets a deliberately odd size so the number it
looks for cannot have come from a default, and asserts the rendered
location. Its control names the whole directive rather than the bare
old value: nginx comments render into the config, so the comment above
the directive mentions 50M and a looser arm matched itself.
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