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 {