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>

ingestion/opengoods/jobs/dedup.py

@@ -12,6 +12,7 @@ import sys
 
 import psycopg
 
+from opengoods.cache import bump_cache_epoch
 from opengoods.etl.dedup import dedup_all
 from opengoods.etl.load import default_dsn
 
@@ -23,6 +24,8 @@ def run(args: argparse.Namespace) -> int:
             conn.rollback()
         else:
             conn.commit()
+    if not args.dry_run and summary["merged"]:
+        bump_cache_epoch()
     mode = "dry-run" if args.dry_run else "applied"
     print(f"{mode} groups={summary['groups']} merged={summary['merged']}")
     return 0