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)

go.mod

@@ -5,10 +5,13 @@ go 1.25.0
 require (
 	github.com/BurntSushi/toml v1.5.0
 	github.com/PuerkitoBio/goquery v1.12.0
+	github.com/redis/go-redis/v9 v9.18.0
 )
 
 require (
 	github.com/andybalholm/cascadia v1.3.3 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
+	go.uber.org/atomic v1.11.0 // indirect
 	golang.org/x/net v0.52.0 // indirect
-	golang.org/x/sync v0.20.0 // indirect
 )