Tighten map GeoJSON fallback behavior for large datasets

## Context
The map endpoint already supports bounds filtering and server-side clustering, but direct unfiltered requests can still return the full approved-library GeoJSON payload.

## Scope
- Review whether unbounded high-zoom or no-zoom requests should be limited, clustered, or require bounds.
- Add payload size, query count, and response-time measurements before and after changes.
- Keep old cached pages and direct links from breaking where possible.

## Acceptance criteria
- The endpoint has a clear safeguard for large unbounded responses.
- Tests document behavior for no bounds, bounds, low zoom clustering, and search filters.
- The map still loads correctly on first render.