refactor: address code review feedback for auth and pagination improvements

- Extract duplicated thread/message pagination functions to shared utils module
- Refactor pagination token parsing to use Option combinators instead of defaults
- Split access token generation from assignment for clearer error handling
- Add appservice token collision detection at startup and registration
- Allow appservice re-registration with same token (for config updates)
- Simplify thread relation chunk building using iterator chaining
- Fix saturating_inc edge case in relation queries with explicit filtering
- Add concise comments explaining non-obvious behaviour choices

src/api/router/auth.rs

@@ -355,6 +355,7 @@ async fn find_token(services: &Services, token: Option<&str>) -> Result<Token> {
 		.map_ok(Token::Appservice);
 
 	pin_mut!(user_token, appservice_token);
+	// Returns Ok if either token type succeeds, Err only if both fail
 	match select_ok([Left(user_token), Right(appservice_token)]).await {
 		| Err(e) if !e.is_not_found() => Err(e),
 		| Ok((token, _)) => Ok(token),