Implement queue timeout #45 (#71)

### Description

This PR implements a queue timeout in the lobby server.

* After the time configured in `timeout` has elapsed (default: 3 minutes) since the first player joins the queue, matchmaking will start even if the queue has not reached the `players_per_game` target. Any missing slots will be filled with fake players.
* If the first queued player leaves and other players remain, the timer is recalculated so that matchmaking starts after the `timeout` duration (default: 3 minutes) from the next player’s join time. (After that, the timer continues to be adjusted in the same way whenever the head of the queue changes.)

### Game

Robocraft

### Please confirm

- [x] I am the legal owner or represent the owner of all work submitted
- [x] I consent to my submission being added to this FOSS project
- [x] This PR used LLMs to generate some or all of the code changes

Reviewed-on: https://git.ngram.ca/OpenJam/rc-servers/pulls/71
Reviewed-by: NGnius <ngniusness@gmail.com>
Co-authored-by: MaxSignal <kastera58@gmail.com>
Co-committed-by: MaxSignal <kastera58@gmail.com>

rc_core/src/persist/combat.rs

@@ -619,6 +619,7 @@ fn default_multiplayer() -> super::MultiplayerConfig {
     super::MultiplayerConfig {
         players_per_game: 2,
         enabled: true,
+        autostart_after_s: 180,
         network: super::multiplayer::default_net_conf(),
         fakes: super::multiplayer::default_fake_users(),
         battle_arena: super::multiplayer::default_ba_conf(),

rc_core/src/persist/config/cubes_json.rs

@@ -403,6 +403,10 @@ impl <C: Clone + Send> super::ConfigProvider<C> for CubeConfig {
     fn is_multiplayer_enabled(&self) -> bool {
         self.battle.multiplayer.enabled
     }
+    
+    fn multiplayer_autostart_after(&self) -> std::time::Duration {
+        std::time::Duration::from_secs(self.battle.multiplayer.autostart_after_s)
+    }
 
     fn network_config(&self) -> crate::persist::NetworkConf {
         self.battle.multiplayer.network.clone()

rc_core/src/persist/config/traits.rs

@@ -27,6 +27,7 @@ pub trait ConfigProvider<C: Clone> {
     fn singleplayer_details(&self) -> SingleplayerConfig;
     fn players_per_game(&self) -> usize;
     fn is_multiplayer_enabled(&self) -> bool;
+    fn multiplayer_autostart_after(&self) -> std::time::Duration;
     // FIXME don't use serializable types in traits
     fn network_config(&self) -> crate::persist::NetworkConf;
     fn maps(&self) -> std::collections::HashMap<GameMap, MapConfig>;

rc_core/src/persist/multiplayer.rs

@@ -4,6 +4,7 @@ use serde::{Serialize, Deserialize};
 pub struct MultiplayerConfig {
     pub players_per_game: usize,
     pub enabled: bool,
+    pub autostart_after_s: u64,
     #[serde(default = "default_net_conf")]
     pub network: NetworkConf,
     #[serde(default = "default_fake_users")]