refactor: use $facet to merge services count query into single pipeline

[james-cross]

Summary

The services API was running two separate MongoDB aggregate calls per request: one for results and one for count. This merges them into a single round trip using $facet, and removes dead code that was adding pagination stages only to immediately strip them off.

Changes

• Replace two aggregate() calls with a single $facet pipeline that returns both results and count
• Remove $skip/$limit stages that were added to the pipeline then spliced off before execution
• Remove unused $skip and $limit from the AggregationStage type
• Add comments explaining why pagination happens in JS (services and accommodation must be interleaved by distance from separate collections)
• Document the MAX_SERVICES_FETCH_LIMIT constant with its rationale

Testing

• All 466 unit tests pass
• TypeScript type-check passes with no errors
• ESLint passes with no warnings or errors
• No behavioural changes to the API response format or pagination logic