feat: Valkey cache for search results

- Add internal/cache package using go-redis/v9 (Valkey-compatible)
- Cache keys are deterministic SHA-256 hashes of search parameters
- Cache wraps the Search() method: check cache → miss → execute → store
- Gracefully disabled if Valkey is unreachable or unconfigured
- Configurable TTL (default 5m), address, password, and DB index
- Environment variable overrides: VALKEY_ADDRESS, VALKEY_PASSWORD,
  VALKEY_DB, VALKEY_CACHE_TTL
- Structured JSON logging via slog throughout cache layer
- Refactored service.go: extract executeSearch() from Search() for clarity
- Update config.example.toml with [cache] section
- Add cache package tests (key generation, nop behavior)

internal/config/config.go

@@ -11,9 +11,10 @@ import (
 
 // Config is the top-level configuration for the gosearch service.
 type Config struct {
-	Server  ServerConfig  `toml:"server"`
+	Server   ServerConfig   `toml:"server"`
 	Upstream UpstreamConfig `toml:"upstream"`
-	Engines EnginesConfig `toml:"engines"`
+	Engines  EnginesConfig  `toml:"engines"`
+	Cache    CacheConfig    `toml:"cache"`
 }
 
 type ServerConfig struct {
@@ -26,11 +27,19 @@ type UpstreamConfig struct {
 }
 
 type EnginesConfig struct {
-	LocalPorted []string   `toml:"local_ported"`
+	LocalPorted []string    `toml:"local_ported"`
 	Brave       BraveConfig `toml:"brave"`
 	Qwant       QwantConfig `toml:"qwant"`
 }
 
+// CacheConfig holds Valkey/Redis cache settings.
+type CacheConfig struct {
+	Address    string `toml:"address"`     // Valkey server address (e.g. "localhost:6379")
+	Password   string `toml:"password"`    // Auth password (empty = none)
+	DB         int    `toml:"db"`          // Database index (default 0)
+	DefaultTTL string `toml:"default_ttl"` // Cache TTL (e.g. "5m", default "5m")
+}
+
 type BraveConfig struct {
 	APIKey       string `toml:"api_key"`
 	AccessToken  string `toml:"access_token"`
@@ -71,6 +80,10 @@ func defaultConfig() *Config {
 				ResultsPerPage: 10,
 			},
 		},
+		Cache: CacheConfig{
+			DB:         0,
+			DefaultTTL: "5m",
+		},
 	}
 }
 
@@ -99,6 +112,18 @@ func applyEnvOverrides(cfg *Config) {
 	if v := os.Getenv("BRAVE_ACCESS_TOKEN"); v != "" {
 		cfg.Engines.Brave.AccessToken = v
 	}
+	if v := os.Getenv("VALKEY_ADDRESS"); v != "" {
+		cfg.Cache.Address = v
+	}
+	if v := os.Getenv("VALKEY_PASSWORD"); v != "" {
+		cfg.Cache.Password = v
+	}
+	if v := os.Getenv("VALKEY_DB"); v != "" {
+		fmt.Sscanf(v, "%d", &cfg.Cache.DB)
+	}
+	if v := os.Getenv("VALKEY_CACHE_TTL"); v != "" {
+		cfg.Cache.DefaultTTL = v
+	}
 }
 
 // HTTPTimeout parses the configured timeout string into a time.Duration.
@@ -114,6 +139,14 @@ func (c *Config) LocalPortedCSV() string {
 	return strings.Join(c.Engines.LocalPorted, ",")
 }
 
+// CacheTTL parses the configured cache TTL string into a time.Duration.
+func (c *Config) CacheTTL() time.Duration {
+	if d, err := time.ParseDuration(c.Cache.DefaultTTL); err == nil && d > 0 {
+		return d
+	}
+	return 5 * time.Minute
+}
+
 func splitCSV(s string) []string {
 	if s == "" {
 		return nil