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/import_bypos.py

@@ -25,6 +25,7 @@ from collections.abc import Iterator
 import psycopg
 
 from opengoods.adapters.bypos import transform_bypos
+from opengoods.cache import bump_cache_epoch
 from opengoods.etl.load import default_dsn, ensure_bypos_source, load_bypos_record_safe
 
 
@@ -59,6 +60,8 @@ def run(args: argparse.Namespace) -> int:
             else:
                 errored += 1
         conn.commit()
+    if loaded:
+        bump_cache_epoch()
     print(f"loaded={loaded} skipped={skipped} errored={errored}")
     return 0