Fixes #813: Application services were unable to work because their sender_localpart
user was never created in the database, preventing authentication.

This fix ensures the appservice user account is created when:
- The server starts up and loads existing appservices from the database
- A new appservice is registered via the admin command

Additionally, if an appservice user has been accidentally deactivated, it will be
automatically reactivated when the appservice starts.

The solution centralises all appservice startup logic into a single `start_appservice`
helper method, eliminating code duplication between the registration and startup paths.

@@ -64,0 +64,4 @@
+/// Parse a pagination token, trying ShortEventId first, then falling back to
+/// PduCount
+async fn parse_pagination_token(
+	_services: &Services,

@@ -64,0 +69,4 @@
+	token: Option<&str>,
+	default: PduCount,
+) -> Result<PduCount> {
+	let Some(token) = token else {

@@ -88,2 +117,2 @@
-		.transpose()?
-		.unwrap_or_else(|| match body.dir {
+	let from: PduCount =
+		parse_pagination_token(&services, room_id, body.from.as_deref(), match body.dir {

@@ -93,2 +122,3 @@
+		.await?;
 
-	let to: Option<PduCount> = body.to.as_deref().map(str::parse).flat_ok();
+	let to: Option<PduCount> = if let Some(to_str) = body.to.as_deref() {

@@ -22,1 +26,4 @@
 
+/// Parse a pagination token, trying ShortEventId first, then falling back to
+/// PduCount
+async fn parse_pagination_token(

@@ -23,0 +53,4 @@
+}
+
+/// Convert a PduCount to a token string (using the underlying ShortEventId)
+fn count_to_token(count: PduCount) -> String {

@@ -116,3 +150,1 @@
-			| Direction::Forward => PduCount::min(),
-			| Direction::Backward => PduCount::max(),
-		});
+	let start: PduCount = parse_pagination_token(services, room_id, from, match dir {

@@ -119,2 +154,3 @@
+	.await?;
 
-	let to: Option<PduCount> = to.map(str::parse).flat_ok();
+	let to: Option<PduCount> = if let Some(to_str) = to {

@@ -162,0 +232,4 @@
+			| Direction::Forward => events.last(),
+			| Direction::Backward => events.first(),
+		}
+		.map(|(count, _)| count_to_token(*count))

@@ -162,0 +238,4 @@
+	};
+
+	// Build the response chunk with thread root if needed
+	let chunk: Vec<_> = if let Some(root) = root_event {

@@ -345,0 +355,4 @@
+		.map_ok(Token::Appservice);
+
+	pin_mut!(user_token, appservice_token);
+	match select_ok([Left(user_token), Right(appservice_token)]).await {

@@ -56,0 +57,4 @@
+				if let Ok((user_id, device_id)) = self
+					.services
+					.users
+					.find_from_token(&registration.as_token)

@@ -114,3 +183,3 @@
 	}
 
-	pub async fn find_from_token(&self, token: &str) -> Option<RegistrationInfo> {
+	pub async fn find_from_token(&self, token: &str) -> Result<RegistrationInfo> {

@@ -64,3 +65,3 @@
 		let mut current = ArrayVec::<u8, 16>::new();
 		current.extend(target.to_be_bytes());
-		current.extend(from.saturating_inc(dir).into_unsigned().to_be_bytes());
+		current.extend(from_unsigned.to_be_bytes());

@@ -410,0 +417,4 @@
+			.await
+			.is_ok()
+		{
+			let new_token = utils::random_string(32);