feat: add timeline API with schedule-aware enrollment aggregation

Implements POST /api/timeline endpoint that aggregates enrollment by
subject over 15-minute slots, filtering courses by their actual meeting
times. Includes ISR-style schedule cache with hourly background refresh
using stale-while-revalidate pattern, database indexes for efficient
queries, and frontend refactor to dynamically discover subjects from API.

src/web/mod.rs

@@ -11,7 +11,9 @@ pub mod calendar;
 pub mod encoding;
 pub mod extractors;
 pub mod routes;
+pub mod schedule_cache;
 pub mod session_cache;
+pub mod timeline;
 pub mod ws;
 
 pub use routes::*;