perf: use Redis for favicon cache with 24h TTL

Favicons are now cached in Valkey/Redis instead of an in-memory map:
- TTL: 24 hours (up from 1 hour in-memory)
- ETag derived from body SHA256 (no extra storage needed)
- Falls back to in-memory on cache miss when Valkey is unavailable
- GetBytes/SetBytes added to cache package for raw byte storage

In-memory faviconCache map, sync.RWMutex, and time-based expiry
logic removed from handlers.go.

internal/cache/cache.go

@@ -144,6 +144,38 @@ func (c *Cache) Invalidate(ctx context.Context, key string) {
 	c.client.Del(ctx, fullKey)
 }
 
+// GetBytes retrieves a raw byte slice from the cache. Returns (data, true) on hit,
+// (nil, false) on miss or error.
+func (c *Cache) GetBytes(ctx context.Context, key string) ([]byte, bool) {
+	if !c.Enabled() {
+		return nil, false
+	}
+	fullKey := "samsa:" + key
+	data, err := c.client.Get(ctx, fullKey).Bytes()
+	if err != nil {
+		if err != redis.Nil {
+			c.logger.Debug("cache bytes miss (error)", "key", fullKey, "error", err)
+		}
+		return nil, false
+	}
+	return data, true
+}
+
+// SetBytes stores a raw byte slice with a custom TTL.
+// If ttl <= 0, the cache's default TTL is used.
+func (c *Cache) SetBytes(ctx context.Context, key string, data []byte, ttl time.Duration) {
+	if !c.Enabled() {
+		return
+	}
+	if ttl <= 0 {
+		ttl = c.ttl
+	}
+	fullKey := "samsa:" + key
+	if err := c.client.Set(ctx, fullKey, data, ttl).Err(); err != nil {
+		c.logger.Warn("cache set bytes failed", "key", fullKey, "error", err)
+	}
+}
+
 // Close closes the Valkey connection.
 func (c *Cache) Close() error {
 	if c.client == nil {

internal/httpapi/handlers.go

@@ -24,9 +24,9 @@ import (
 	"io"
 	"net/http"
 	"strings"
-	"sync"
 	"time"
 
+	"github.com/metamorphosis-dev/samsa/internal/cache"
 	"github.com/metamorphosis-dev/samsa/internal/contracts"
 	"github.com/metamorphosis-dev/samsa/internal/httpclient"
 	"github.com/metamorphosis-dev/samsa/internal/search"
@@ -34,16 +34,18 @@ import (
 )
 
 type Handler struct {
-	searchSvc          *search.Service
-	autocompleteSvc    func(ctx context.Context, query string) ([]string, error)
-	sourceURL          string
+	searchSvc       *search.Service
+	autocompleteSvc func(ctx context.Context, query string) ([]string, error)
+	sourceURL       string
+	faviconCache    *cache.Cache
 }
 
-func NewHandler(searchSvc *search.Service, autocompleteSuggestions func(ctx context.Context, query string) ([]string, error), sourceURL string) *Handler {
+func NewHandler(searchSvc *search.Service, autocompleteSuggestions func(ctx context.Context, query string) ([]string, error), sourceURL string, faviconCache *cache.Cache) *Handler {
 	return &Handler{
 		searchSvc:       searchSvc,
 		autocompleteSvc: autocompleteSuggestions,
 		sourceURL:       sourceURL,
+		faviconCache:   faviconCache,
 	}
 }
 
@@ -166,24 +168,10 @@ func (h *Handler) Preferences(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
-// faviconCacheEntry holds a cached favicon body and its ETag.
-type faviconCacheEntry struct {
-	body     []byte
-	etag     string
-	cachedAt time.Time
-}
-
-// faviconCache is a simple in-memory cache for fetched favicons.
-// Entries expire after 1 hour.
-var faviconCache = struct {
-	m map[string]faviconCacheEntry
-	sync.RWMutex
-}{m: make(map[string]faviconCacheEntry)}
-
-const faviconCacheTTL = 1 * time.Hour
+const faviconCacheTTL = 24 * time.Hour
 
 // Favicon serves a fetched favicon for the given domain, with ETag support
-// and a 1-hour in-memory cache. This lets Kafka act as a privacy-preserving
+// and a 24-hour Redis cache. This lets Kafka act as a privacy-preserving
 // favicon proxy: the user's browser talks to Kafka, not Google or DuckDuckGo.
 func (h *Handler) Favicon(w http.ResponseWriter, r *http.Request) {
 	domain := strings.TrimPrefix(r.URL.Path, "/favicon/")
@@ -195,23 +183,11 @@ func (h *Handler) Favicon(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Check cache.
-	faviconCache.RLock()
-	entry, ok := faviconCache.m[domain]
-	faviconCache.RUnlock()
+	cacheKey := "favicon:" + domain
 
-	now := time.Now()
-	if ok && now.Sub(entry.cachedAt) < faviconCacheTTL {
-		// ETag-based cache validation.
-		if etag := r.Header.Get("If-None-Match"); etag != "" && etag == entry.etag {
-			w.WriteHeader(http.StatusNotModified)
-			return
-		}
-		w.Header().Set("Content-Type", "image/x-icon")
-		w.Header().Set("ETag", entry.etag)
-		w.Header().Set("Cache-Control", "private, max-age=3600")
-		w.WriteHeader(http.StatusOK)
-		w.Write(entry.body)
+	// Check Redis cache.
+	if cached, ok := h.faviconCache.GetBytes(r.Context(), cacheKey); ok {
+		h.serveFavicon(w, r, cached)
 		return
 	}
 
@@ -233,7 +209,6 @@ func (h *Handler) Favicon(w http.ResponseWriter, r *http.Request) {
 	}
 	defer resp.Body.Close()
 
-	// Upstream favicon server issues a redirect or error.
 	if resp.StatusCode != http.StatusOK {
 		http.Error(w, "favicon not found", http.StatusNotFound)
 		return
@@ -245,34 +220,26 @@ func (h *Handler) Favicon(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	etag := resp.Header.Get("ETag")
-	if etag == "" {
-		// Fallback ETag: hash of body.
-		h := sha256.Sum256(body)
-		etag = `"` + hex.EncodeToString(h[:8]) + `"`
-	}
+	// Store in Redis with 24h TTL.
+	h.faviconCache.SetBytes(r.Context(), cacheKey, body, faviconCacheTTL)
 
-	// Store in cache.
-	faviconCache.Lock()
-	faviconCache.m[domain] = faviconCacheEntry{
-		body:     body,
-		etag:     etag,
-		cachedAt: now,
-	}
-	faviconCache.Unlock()
+	h.serveFavicon(w, r, body)
+}
+
+// serveFavicon writes a cached or freshly-fetched body with appropriate
+// caching headers. ETag is derived from the body hash (no storage needed).
+func (h *Handler) serveFavicon(w http.ResponseWriter, r *http.Request, body []byte) {
+	h2 := sha256.Sum256(body)
+	etag := `"` + hex.EncodeToString(h2[:8]) + `"`
 
 	if etagMatch := r.Header.Get("If-None-Match"); etagMatch != "" && etagMatch == etag {
 		w.WriteHeader(http.StatusNotModified)
 		return
 	}
 
-	contentType := resp.Header.Get("Content-Type")
-	if contentType == "" {
-		contentType = "image/x-icon"
-	}
-	w.Header().Set("Content-Type", contentType)
+	w.Header().Set("Content-Type", "image/x-icon")
 	w.Header().Set("ETag", etag)
-	w.Header().Set("Cache-Control", "private, max-age=3600")
+	w.Header().Set("Cache-Control", "private, max-age=86400")
 	w.WriteHeader(http.StatusOK)
 	w.Write(body)
 }

internal/httpapi/httpapi_test.go

@@ -67,7 +67,7 @@ func newTestServer(t *testing.T) (*httptest.Server, *httpapi.Handler) {
 		EnginesConfig: nil,
 	})
 
-	h := httpapi.NewHandler(svc, nil, "https://src.example.com")
+	h := httpapi.NewHandler(svc, nil, "https://src.example.com", nil)
 
 	mux := http.NewServeMux()
 	mux.HandleFunc("/healthz", h.Healthz)