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/update_off.py

@@ -17,6 +17,7 @@ import sys
 import psycopg
 
 from opengoods.adapters.openfoodfacts import SOURCE_NAME, OpenFoodFactsAdapter
+from opengoods.cache import bump_cache_epoch
 from opengoods.etl.load import default_dsn, ensure_source, load_record_safe
 from opengoods.etl.state import get_watermark, set_watermark
 from opengoods.etl.transform import transform
@@ -49,6 +50,8 @@ def run(args: argparse.Namespace) -> int:
             stats={"loaded": loaded, "skipped": skipped, "errored": errored, "since": since},
         )
         conn.commit()
+    if loaded:
+        bump_cache_epoch()
     print(
         f"since={since} loaded={loaded} skipped={skipped} "
         f"errored={errored} watermark={high_watermark}"