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/tests/test_cache.py

@@ -0,0 +1,44 @@
+"""Tests for the best-effort API cache invalidation signal."""
+
+from __future__ import annotations
+
+import sys
+
+import opengoods.cache as cache
+
+
+def test_disabled_when_no_url(monkeypatch):
+    monkeypatch.delenv("OPENGOODS_REDIS_URL", raising=False)
+    assert cache.default_redis_url() is None
+    assert cache.bump_cache_epoch() is False
+
+
+def test_bump_fails_open_when_unreachable(monkeypatch):
+    # An unroutable URL must not raise; the run still succeeds.
+    monkeypatch.setenv("OPENGOODS_REDIS_URL", "redis://127.0.0.1:1/0")
+    assert cache.bump_cache_epoch() is False
+
+
+def test_bump_increments_epoch(monkeypatch):
+    calls = {}
+
+    class FakeClient:
+        def incr(self, key):
+            calls["key"] = key
+            return 7
+
+        def close(self):
+            calls["closed"] = True
+
+    class FakeRedis:
+        @staticmethod
+        def from_url(url, **kwargs):
+            calls["url"] = url
+            return FakeClient()
+
+    monkeypatch.setitem(sys.modules, "redis", type("M", (), {"Redis": FakeRedis}))
+
+    assert cache.bump_cache_epoch("redis://example:6379/0") is True
+    assert calls["key"] == "og:cache:epoch"
+    assert calls["url"] == "redis://example:6379/0"
+    assert calls["closed"] is True