[Audit][P1] Harden event rendering and query performance

## Context

Malformed event dates can terminate rendering, while filter construction repeatedly loads all events and assumes optional metadata is present.

Audit findings: 17, 32, and the event-related part of 39.

## Scope

- Validate and normalize event date, time, country, speaker, and organisation data.
- Prevent one malformed event from breaking a listing or single page.
- Replace repeated unbounded filter queries with a bounded or aggregated approach.

## Done when

- Empty or invalid event metadata renders a safe fallback.
- Event listings do not issue one unbounded query per filter dimension.
- Single-event and event-list views share tested normalization behavior.