feat: cache GET /api/v1/stats endpoint data in Redis

[Bellamzy]

feat: cache GET /api/v1/stats endpoint data in Redis

Summary

Implements the cache-aside pattern for the GET /api/v1/stats endpoint to reduce repeated
database aggregation queries.

Changes

backend/src/redis_cache.rs

• Added STATS_CACHE_TTL = 30 (30-second TTL)
• Added STATS_CACHE_PATTERN = "stats:*" for bulk invalidation
• Added stats_cache_key(category, status) — encodes filter params into a Redis key (e.g.
  stats:sports:active, stats:all:all)
• Added invalidate_stats_cache() method
• Added unit tests for key generation, uniqueness, TTL, and invalidation

backend/src/routes/v1.rs

• get_stats: check Redis first → on hit return cached value; on miss query DB, cache result for
  30s, return
• ingest_pool_created: invalidates stats cache on new pool (affects total_pools /
  total_value_locked)
• ingest_prediction_placed: invalidates stats cache on new bet (affects total_bets /
  total_value_locked)
• Added unit tests for the cache-aside flow

Behaviour

• Stats are served from Redis for up to 30 seconds after the last write event
• Cache is invalidated immediately when a pool is created or a prediction is placed, so stale
  data is bounded
• Falls back gracefully to the database when Redis is unavailable (fail-open)
  closes feat(backend): cache statistics endpoint data in redis #1145

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