[Security] Harden network input handling and websocket proxying (#7076)

- TCP client connections: reject messages whose declared length is
  negative or exceeds a 1.5MB cap immediately instead of buffering
  indefinitely, preventing unbounded memory growth and the previously
  stuck state caused by negative lengths overflowing the int header.
- ISL connections: apply the same length validation with a 4MB cap,
  disconnecting the peer on an implausible length.
- Websockets: only trust the forwarded client-address header (e.g.
  X-Forwarded-For) when the websocket connection originates from an
  address in security/trusted_sources. Previously any client could
  spoof its address to bypass bans, user limits and rate limiting.

Took 3 minutes

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

servatrice/servatrice.ini.example

@@ -41,6 +41,12 @@ websocket_host=any
 ; The TCP port number servatrice will listen on for websockets clients; default is 4748
 websocket_port=4748
 
+; If websockets are served through a reverse proxy (e.g. nginx), the real client address can be
+; read from an HTTP header such as X-Forwarded-For. The header is ONLY trusted when the websocket
+; connection itself originates from an address listed in security/trusted_sources (default 127.0.0.1,::1),
+; so that clients cannot spoof their address to bypass bans or rate limits.
+web_socket_ip_header=
+
 ; When database is enabled, servatrice writes the server status in the "update" database table; this
 ; setting defines every how many milliseconds servatrice will update its status; default is 15000 (15 secs)
 statusupdate=15000