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/pyproject.toml

@@ -6,6 +6,7 @@ requires-python = ">=3.11"
 dependencies = [
     "httpx>=0.27",
     "psycopg[binary]>=3.2",
+    "redis>=5.0,<6",
 ]
 
 [project.optional-dependencies]