Api Learn Activity Log Emit

Fire-and-forget INSERT of one user activity event.

Accepts JSON: {kind: str, ref_type?: str, ref_id?: str, meta?: dict}.

Validates kind against the closed enum + payload size + meta-as- object before delegating to pg_emit_user_activity. Returns {ok: bool, id: int|None}. The client never retries on ok=false (the writers use fire-and-forget .catch() pattern, see learning-paths.ts / reading-list.ts / glossary.ts).

Loss tolerance is acceptable here because the reading log is a best-effort historical record, not a billing or auth surface (CHAOS §2.1 review). Server-side INSERT failures are captured by Sentry via the routes/api_errors.py middleware.