feat: add adaptive scheduling and scraper admin endpoints

Subjects now have individually calculated scrape intervals based on their
historical change ratio, consecutive zero-change runs, failure counts, and
the current time of day. This reduces unnecessary scrapes during inactive
periods while maintaining responsiveness during peak hours. Includes four
new admin endpoints for monitoring scraper health and scheduling decisions.

src/web/mod.rs

@@ -2,6 +2,7 @@
 
 pub mod admin;
 pub mod admin_rmp;
+pub mod admin_scraper;
 #[cfg(feature = "embed-assets")]
 pub mod assets;
 pub mod auth;