Api Learn Activity Log

Return the authenticated user's most recent activity events.

Used by progress.ts:fetchActivityLog() to render the Reading-Room Ledger's reading-log section. Falls through to the synthesis fallback (progress.ts:deriveReadingLog) on:

• 401 anon caller (require_auth rejects)
• 200 with rows=[] (brownfield pre-v112, no events yet, or PG read failure swallowed by pg_get_user_activity_log)

The reading-log render in progress.ts:buildReadingLog handles both branches uniformly -- rows=[] paints the "No entries yet" empty state.