Implement gist TTL expiry — auto-expire gists after configurable time-to-live

[BigBen-7]

Description

Gists are hyperlocal and time-sensitive by nature. This issue adds a TTL (time-to-live) system where gists automatically expire after a configurable period (default 24 hours). Expired gists should not be returned in GET /gists queries but should remain in the database for audit purposes.

Context

• The gists table already has a created_at column (timestamptz)
• We need to add an expires_at column (timestamptz)
• The CreateGistDto should accept an optional ttlHours field (default: 24, max: 168 = 7 days)
• All geospatial queries must filter out expired gists

Requirements

• Add expires_at column to the Gist entity
• Generate a TypeORM migration for the new column
• Update CreateGistDto to accept optional ttlHours (number, min: 1, max: 168)
• In GistsService.create(), calculate and set expires_at = created_at + ttlHours
• Update GistRepository.findNearby() to add WHERE expires_at > NOW() to all queries
• Update GistRepository.findByGistId() to also filter expired gists
• Add a database index on expires_at for query performance
• Add a scheduled CronJob (NestJS @Cron) that soft-deletes expired gists every hour

Files to Touch

• Backend/src/gists/entities/gist.entity.ts — add expires_at column
• Backend/src/gists/dto/create-gist.dto.ts — add optional ttlHours field
• Backend/src/gists/gists.service.ts — set expires_at on create
• Backend/src/gists/gist.repository.ts — filter expired gists in all queries
• Backend/src/database/migrations/ — new migration for expires_at column
• Backend/src/gists/gists.module.ts — register ScheduleModule if adding cron
• Backend/package.json — add @nestjs/schedule if not present

Migration SQL (reference)

ALTER TABLE gists ADD COLUMN expires_at TIMESTAMPTZ NOT NULL DEFAULT NOW() + INTERVAL '24 hours';
CREATE INDEX idx_gists_expires_at ON gists (expires_at);

Acceptance Criteria

• expires_at column exists in the database after migration
• POST /gists with ttlHours: 2 sets expiry 2 hours from now
• GET /gists does NOT return expired gists
• GET /gists/:id returns 404 for expired gists
• Cron job logs when it cleans up expired gists
• Unit tests cover expiry filtering logic

Complexity: 300 points