fix(auth): prevent token collisions and optimise lookups

Ensures access tokens are unique across both user and appservice tables to
prevent authentication ambiguity and potential security issues.

Changes:
- On startup, automatically logout any user devices using tokens that
  conflict with appservice tokens (resolves in favour of appservices)
  and log a warning with affected user/device details
- When creating new user tokens, check for conflicts with appservice tokens
  and generate a new token if a collision would occur
- When registering new appservices, reject registration if the token is
  already in use by a user device
- Use futures::select_ok to race token lookups concurrently for better
  performance (adapted from tuwunel commit 066097a8)

This fix-forward approach resolves existing token collisions on startup
whilst preventing new ones from being created, without breaking existing
valid authentications.

The find_token optimisation is adapted from tuwunel (matrix-construct/tuwunel)
commit 066097a8: "Optimize user and appservice token queries" by Jason Volk.

src/api/router/auth.rs

@@ -5,6 +5,14 @@ use axum_extra::{
 	typed_header::TypedHeaderRejectionReason,
 };
 use conduwuit::{Err, Error, Result, debug_error, err, warn};
+use futures::{
+	TryFutureExt,
+	future::{
+		Either::{Left, Right},
+		select_ok,
+	},
+	pin_mut,
+};
 use ruma::{
 	CanonicalJsonObject, CanonicalJsonValue, OwnedDeviceId, OwnedServerName, OwnedUserId, UserId,
 	api::{
@@ -54,17 +62,7 @@ pub(super) async fn auth(
 		| None => request.query.access_token.as_deref(),
 	};
 
-	let token = if let Some(token) = token {
-		match services.appservice.find_from_token(token).await {
-			| Some(reg_info) => Token::Appservice(Box::new(reg_info)),
-			| _ => match services.users.find_from_token(token).await {
-				| Ok((user_id, device_id)) => Token::User((user_id, device_id)),
-				| _ => Token::Invalid,
-			},
-		}
-	} else {
-		Token::None
-	};
+	let token = find_token(services, token).await?;
 
 	if metadata.authentication == AuthScheme::None {
 		match metadata {
@@ -342,3 +340,24 @@ async fn parse_x_matrix(request: &mut Request) -> Result<XMatrix> {
 
 	Ok(x_matrix)
 }
+
+async fn find_token(services: &Services, token: Option<&str>) -> Result<Token> {
+	let Some(token) = token else {
+		return Ok(Token::None);
+	};
+
+	let user_token = services.users.find_from_token(token).map_ok(Token::User);
+
+	let appservice_token = services
+		.appservice
+		.find_from_token(token)
+		.map_ok(Box::new)
+		.map_ok(Token::Appservice);
+
+	pin_mut!(user_token, appservice_token);
+	match select_ok([Left(user_token), Right(appservice_token)]).await {
+		| Err(e) if !e.is_not_found() => Err(e),
+		| Ok((token, _)) => Ok(token),
+		| _ => Ok(Token::Invalid),
+	}
+}