bug: harden website chat sends (auth, 2000-char cap, no relay of rejected messages)

[lukepolo]

Hardens website chat sends: a message the room refused was still relayed into the live game over RCON, so any signed-in socket could print into any match.

• Relay to the game server only when sendMessageToChat accepted the message
• Re-check room membership on every send, not just on join (a player swapped out mid-match, a DM after an unfriend)
• Keep strangers out of matchmaking match chat: is_organizer is NULL there, so the old === false check let everyone in
• Ignore non-string, empty and unauthenticated sends; refuse over 2000 chars; clamp lines relayed to the game to 240 code points
• Fall back to the player's own role or name when only one of them is cached; store source and a string steam_id on every message
• New socket events: lobby:chat takes an optional requestId; chat:ack {requestId, messageId} and chat:error {code, max?, requestId?} out

Merge/deploy: 🔒 security. Pairs with 5stackgg/web#610.

Tests: every fix fails its test when reverted: refused-send relay, null-organizer join and send, per-send membership (lineup swap, DM after unfriend), role/name fallback, non-string crash, unauthenticated socket, 2000 cap, RCON clamp, BigInt steam id. Based on DEAFCS c76bba393