Authenticate through Mojang's session server; add version manifest

Replace `auth` (which received the player's Minecraft access token) with
the vanilla online-mode flow, so the token never reaches this server:

- auth_challenge: validate the username and return a random one-time
  server id
- the client calls Mojang's session `join` with its token and that id
- auth_verify: confirm the join with Mojang's `hasJoined` and log the
  player in

Add GET /versions returning {"supported": [...], "deprecated": [...]}
from the SUPPORTED_VERSIONS / DEPRECATED_VERSIONS env vars (defaults:
0.1.0-beta3 supported), exposed in compose.yaml.

Refs saturnclientmc/saturnclient#7

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Cargo.toml

@@ -5,7 +5,8 @@ edition = "2024"
 
 [dependencies]
 axum = { version = "0.8.9", features = ["ws"] }
-reqwest = { version = "0.13.2", features = ["json"] }
+rand = "0.9"
+reqwest = { version = "0.13.2", features = ["json", "query"] }
 serde = { version = "1.0.228", features = ["serde_derive"] }
 serde_json = "1.0.149"
 session-rs = { version = "0.2.1", default-features = false, features = ["axum"] }