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.
Implements a canvas-based timeline view with D3 scales showing class
counts across subjects. Features drag-to-pan, mouse wheel zoom, subject
filtering, hover tooltips, and smooth animations. Timeline auto-follows
current time and supports keyboard navigation.
