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)

config.example.toml

@@ -29,3 +29,15 @@ access_token = ""
 # Qwant category: "web" or "web-lite" (default: "web-lite")
 category = "web-lite"
 results_per_page = 10
+
+[cache]
+# Valkey/Redis cache for search results.
+# Leave address empty to disable caching entirely.
+# Env: VALKEY_ADDRESS
+address = ""
+# Env: VALKEY_PASSWORD
+password = ""
+# Database index (env: VALKEY_DB)
+db = 0
+# Cache TTL for search results (env: VALKEY_CACHE_TTL)
+default_ttl = "5m"