fix(gists): dynamic radius with ST_DWithin and distance in response

[mogbonjubolaolasunkanmi-art]

Summary

Fixes #607 — closes the gaps in the findNearby spatial query and adds distanceMeters to each returned gist.

Changes

gist.entity.ts

• Added @Index('idx_gists_location_geography', { synchronize: false }) to document the GIST spatial index (created via migration, not auto-sync).
• Added distanceMeters?: number field to Gist type (populated at query time, not persisted).

gist.repository.ts

• Added ::geography cast to both the ST_DWithin filter and the ST_Distance select in findNearby().
• Added AND g.expires_at > NOW() to filter out expired gists.
• Changed ORDER BY from g.created_at DESC to distance_meters ASC, g.created_at DESC (nearest-first, ties broken by recency).
• Maps raw distance_meters string from Postgres → distanceMeters: number on each result object.

gists.service.ts

• Removed duplicate import { Injectable, Logger } statement.
• Removed dead return this.gistRepository.create(…) block after an existing throw err (unreachable code).

Acceptance Criteria Checklist

• GET /gists?lat=6.5244&lon=3.3792&radius=1000 returns gists within 1 km
• Each gist in the response includes distanceMeters
• Results are sorted nearest-first
• radius > 5000 returns 400 (enforced by @Max(5000) on QueryGistsDto)
• radius < 50 returns 400 (enforced by @Min(50) on QueryGistsDto)
• Spatial index idx_gists_location_geography (GIST on location::geography) documented — EXPLAIN ANALYZE will confirm index usage once the DB is running

EXPLAIN ANALYZE note

The GIST index on location::geography is the standard PostGIS index for ST_DWithin / ST_Distance with the geography type. Running:

EXPLAIN ANALYZE
SELECT * FROM gists
WHERE ST_DWithin(location::geography,
  ST_SetSRID(ST_MakePoint(3.3792, 6.5244), 4326)::geography, 1000)
AND expires_at > NOW();

should show Index Scan using idx_gists_location_geography in the plan.