feat(api): Redis read cache for product details and search

Stage-0 caching from docs/scalability.md. The Go API now caches hot
product-detail and search-result reads in Redis with a fail-open,
epoch-versioned scheme; Python ingestion bumps the epoch after a write
run to invalidate the cache globally in O(1).

- api/internal/cache: fail-open Cache (GetJSON/SetJSON) namespaced by an
  epoch counter (og:cache:epoch); disabled when Redis is unconfigured.
- store: ProductByID/ProductByGTIN (24h TTL) and SearchProducts (1h TTL)
  read-through the cache via WithCache.
- ingestion: bump_cache_epoch() called after update_off/seed_off/
  import_bypos/dedup commits when rows changed; best-effort, never fails
  a run. Adds redis dependency.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

api/cmd/server/main.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/jackc/pgx/v5/pgxpool"
 
+	"github.com/baicai2026-baicai/goods/api/internal/cache"
 	"github.com/baicai2026-baicai/goods/api/internal/config"
 	"github.com/baicai2026-baicai/goods/api/internal/handler"
 	"github.com/baicai2026-baicai/goods/api/internal/publicweb"
@@ -36,7 +37,12 @@ func main() {
 	if !limiter.Enabled() {
 		log.Print("warning: Redis not configured; public API rate limiting disabled")
 	}
-	h := handler.New(store.New(pool), publicweb.Dist()).
+
+	readCache := cache.New(cfg.RedisURL)
+	if !readCache.Enabled() {
+		log.Print("warning: Redis not configured; public API read cache disabled")
+	}
+	h := handler.New(store.New(pool).WithCache(readCache), publicweb.Dist()).
 		WithRateLimit(limiter, cfg.AnonRateLimitPerMin).
 		WithQuotas(cfg.AnonTotalQuota, cfg.RegisteredRateLimitPerMin, cfg.RegisteredQuotaTotal)