Harden four things the audit turned up

**The login rate limiter could be sidestepped.** X-Forwarded-For is a list each
hop appends to, and nginx's $proxy_add_x_forwarded_for appends ours — so a
client sending "X-Forwarded-For: 1.2.3.4" arrives as "1.2.3.4, <their real
address>". Reading the leftmost entry, as we did, handed the caller a
rate-limit key they could change per request: unlimited password guessing
against a deployment that looks correctly configured. Read from the right
instead, skip hops that are themselves trusted proxies, and believe the header
only when the peer is one (loopback and the private ranges by default,
TRUSTED_PROXIES to be explicit).

**The upload cap was a suggestion.** It read content-length, which a chunked
request simply omits. Count the bytes through a stream, as the image proxy
already does.

**App password secrets were drawn with a modulo.** 256 is not a multiple of 33,
so the first 25 characters of the alphabet came up on 8 byte values and the
last 8 on only 7. Rejection sampling instead. The test weighs the whole tail of
the alphabet rather than single characters, because a 7/8 skew is invisible
per character against the noise — and it does fail when the bias is put back.

**Upstream headers were relayed wholesale.** Anything the mail server set —
cookies, auth challenges, CORS grants — landed on our origin, where it means
something else. Allowlist what is actually wanted.

README.md

@@ -141,7 +141,8 @@ All configuration is via environment variables (see `.env.example`):
 | `STALWART_URL` | `https://mail.example.com` | Base URL of Stalwart; the JMAP session is discovered at `/.well-known/jmap` |
 | `APP_SECRET` | *(required in production)* | Secret used to derive session encryption keys |
 | `PORT` / `HOST` | `8080` / `0.0.0.0` | Listen address |
-| `TRUST_PROXY` | `1` | Honour `X-Forwarded-*` from a reverse proxy |
+| `TRUST_PROXY` | `1` | Honour `X-Forwarded-*`, but only from a peer listed in `TRUSTED_PROXIES` |
+| `TRUSTED_PROXIES` | *(loopback + private ranges)* | Comma-separated CIDRs or addresses whose forwarding headers are believed. Anything else is attributed by its socket address, whatever it claims |
 | `SECURE_COOKIES` | `auto` | `auto` (Secure on https), `1`, or `0` for plain-HTTP dev |
 | `SESSION_TTL` / `SESSION_REMEMBER_TTL` | `43200` / `2592000` | Idle session lifetime (seconds), with/without "keep me signed in" |
 | `SESSION_FILE` | *(unset)* | Persist sessions across restarts (ciphertext only) |