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>

api/internal/cache/cache.go

@@ -0,0 +1,126 @@
+// Package cache is a Redis-backed, fail-open read cache for the public API.
+//
+// It caches hot product details and search results so repeated reads avoid
+// PostgreSQL. Like the ratelimit package, every operation fails open: if Redis
+// is unavailable or misconfigured the caller simply falls back to the database,
+// so the cache can never take the API down or serve stale data after Redis loss.
+//
+// Invalidation is global and O(1): keys are namespaced by an epoch counter
+// stored in Redis (og:cache:epoch). The Python ingestion bumps that counter
+// after a write run, which logically invalidates every cached entry at once
+// while old keys age out via their TTL. The epoch is read at most once per
+// refresh interval per process, so it adds no per-request round trip.
+package cache
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"log"
+	"strconv"
+	"sync"
+	"time"
+
+	"github.com/redis/go-redis/v9"
+)
+
+// epochKey is the Redis key holding the global cache generation counter.
+const epochKey = "og:cache:epoch"
+
+// epochRefresh bounds how often a process re-reads the epoch from Redis.
+const epochRefresh = 10 * time.Second
+
+// opTimeout caps any single Redis operation so a slow backend never blocks a
+// request beyond this; on timeout the cache fails open.
+const opTimeout = 150 * time.Millisecond
+
+// Cache wraps a Redis client. A nil-backed Cache (Redis unconfigured) disables
+// caching: every Get misses and every Set is a no-op.
+type Cache struct {
+	rdb *redis.Client
+
+	mu         sync.RWMutex
+	epoch      int64
+	epochSetAt time.Time
+	epochOK    bool
+}
+
+// New builds a Cache from a redis:// URL. On a parse error it logs and returns a
+// disabled (fail-open) cache so the server still boots.
+func New(redisURL string) *Cache {
+	opt, err := redis.ParseURL(redisURL)
+	if err != nil {
+		log.Printf("cache: invalid redis url %q: %v (caching disabled)", redisURL, err)
+		return &Cache{}
+	}
+	return &Cache{rdb: redis.NewClient(opt)}
+}
+
+// Enabled reports whether a Redis backend is configured.
+func (c *Cache) Enabled() bool { return c != nil && c.rdb != nil }
+
+// epochNow returns the current cache generation, reading it from Redis at most
+// once per epochRefresh. On any Redis error it keeps the last known value and
+// throttles re-reads so a down backend cannot slow the hot path.
+func (c *Cache) epochNow(ctx context.Context) int64 {
+	c.mu.RLock()
+	if c.epochOK && time.Since(c.epochSetAt) < epochRefresh {
+		e := c.epoch
+		c.mu.RUnlock()
+		return e
+	}
+	c.mu.RUnlock()
+
+	cctx, cancel := context.WithTimeout(ctx, opTimeout)
+	defer cancel()
+	n, err := c.rdb.Get(cctx, epochKey).Int64()
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	switch {
+	case err == nil:
+		c.epoch = n
+	case errors.Is(err, redis.Nil):
+		c.epoch = 0
+	}
+	c.epochSetAt = time.Now()
+	c.epochOK = true
+	return c.epoch
+}
+
+// key namespaces a logical suffix under the current epoch.
+func (c *Cache) key(ctx context.Context, suffix string) string {
+	return "og:v" + strconv.FormatInt(c.epochNow(ctx), 10) + ":" + suffix
+}
+
+// GetJSON unmarshals the cached value for suffix into dest and reports a hit.
+// Any miss, decode error, or Redis error returns false (fail-open).
+func (c *Cache) GetJSON(ctx context.Context, suffix string, dest any) bool {
+	if !c.Enabled() {
+		return false
+	}
+	k := c.key(ctx, suffix)
+	cctx, cancel := context.WithTimeout(ctx, opTimeout)
+	defer cancel()
+	b, err := c.rdb.Get(cctx, k).Bytes()
+	if err != nil {
+		return false
+	}
+	return json.Unmarshal(b, dest) == nil
+}
+
+// SetJSON stores val (JSON-encoded) for suffix with the given TTL. Best effort:
+// marshal or Redis errors are ignored.
+func (c *Cache) SetJSON(ctx context.Context, suffix string, val any, ttl time.Duration) {
+	if !c.Enabled() {
+		return
+	}
+	b, err := json.Marshal(val)
+	if err != nil {
+		return
+	}
+	k := c.key(ctx, suffix)
+	cctx, cancel := context.WithTimeout(ctx, opTimeout)
+	defer cancel()
+	_ = c.rdb.Set(cctx, k, b, ttl).Err()
+}

api/internal/cache/cache_test.go

@@ -0,0 +1,84 @@
+package cache
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+)
+
+// TestDisabledFailsOpen verifies a Cache without a Redis backend never panics,
+// always misses, and silently drops writes.
+func TestDisabledFailsOpen(t *testing.T) {
+	c := New("not-a-valid-url") // parse error => disabled
+	if c.Enabled() {
+		t.Fatal("expected cache to be disabled for invalid url")
+	}
+	c.SetJSON(context.Background(), "k", map[string]int{"a": 1}, time.Minute)
+	var dst map[string]int
+	if c.GetJSON(context.Background(), "k", &dst) {
+		t.Fatalf("disabled cache must always miss, got %+v", dst)
+	}
+}
+
+func testCache(t *testing.T) *Cache {
+	t.Helper()
+	url := os.Getenv("OPENGOODS_REDIS_URL")
+	if url == "" {
+		url = "redis://localhost:6379/0"
+	}
+	c := New(url)
+	if !c.Enabled() {
+		t.Skip("redis not configured")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+	if err := c.rdb.Ping(ctx).Err(); err != nil {
+		t.Skipf("redis not reachable: %v", err)
+	}
+	return c
+}
+
+// TestRoundTrip stores then reads a value back.
+func TestRoundTrip(t *testing.T) {
+	c := testCache(t)
+	ctx := context.Background()
+	suffix := fmt.Sprintf("test:rt:%d", time.Now().UnixNano())
+
+	c.SetJSON(ctx, suffix, map[string]any{"name": "foo", "n": float64(3)}, time.Minute)
+	got := map[string]any{}
+	if !c.GetJSON(ctx, suffix, &got) {
+		t.Fatal("expected cache hit after set")
+	}
+	if got["name"] != "foo" || got["n"] != float64(3) {
+		t.Fatalf("unexpected payload: %+v", got)
+	}
+}
+
+// TestEpochInvalidation verifies that bumping the epoch counter logically drops
+// every previously cached entry.
+func TestEpochInvalidation(t *testing.T) {
+	c := testCache(t)
+	ctx := context.Background()
+	suffix := fmt.Sprintf("test:epoch:%d", time.Now().UnixNano())
+
+	c.SetJSON(ctx, suffix, map[string]int{"v": 1}, time.Minute)
+	var dst map[string]int
+	if !c.GetJSON(ctx, suffix, &dst) {
+		t.Fatal("expected hit before epoch bump")
+	}
+
+	// Simulate an ingestion write bumping the global epoch.
+	if err := c.rdb.Incr(ctx, epochKey).Err(); err != nil {
+		t.Fatalf("incr epoch: %v", err)
+	}
+	// Force the process to re-read the epoch rather than use its cached value.
+	c.mu.Lock()
+	c.epochOK = false
+	c.mu.Unlock()
+
+	if c.GetJSON(ctx, suffix, &dst) {
+		t.Fatal("entry should be invisible after epoch bump")
+	}
+}