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/cache/cache_test.go

@@ -0,0 +1,77 @@
+package cache
+
+import (
+	"testing"
+
+	"github.com/ashie/gosearch/internal/contracts"
+)
+
+func TestKey_Deterministic(t *testing.T) {
+	req := contracts.SearchRequest{
+		Format:     contracts.FormatJSON,
+		Query:      "kafka metamorphosis",
+		Pageno:     1,
+		Safesearch: 0,
+		Language:   "auto",
+		Engines:    []string{"wikipedia", "braveapi"},
+		Categories: []string{"general"},
+	}
+
+	key1 := Key(req)
+	key2 := Key(req)
+
+	if key1 != key2 {
+		t.Errorf("Key should be deterministic: %q != %q", key1, key2)
+	}
+	if len(key1) != 32 {
+		t.Errorf("expected 32-char key, got %d", len(key1))
+	}
+}
+
+func TestKey_DifferentQueries(t *testing.T) {
+	reqA := contracts.SearchRequest{Query: "kafka", Format: contracts.FormatJSON}
+	reqB := contracts.SearchRequest{Query: "orwell", Format: contracts.FormatJSON}
+
+	if Key(reqA) == Key(reqB) {
+		t.Error("different queries should produce different keys")
+	}
+}
+
+func TestKey_DifferentPageno(t *testing.T) {
+	req1 := contracts.SearchRequest{Query: "test", Pageno: 1}
+	req2 := contracts.SearchRequest{Query: "test", Pageno: 2}
+
+	if Key(req1) == Key(req2) {
+		t.Error("different pageno should produce different keys")
+	}
+}
+
+func TestKey_DifferentEngines(t *testing.T) {
+	req1 := contracts.SearchRequest{Query: "test", Engines: []string{"wikipedia"}}
+	req2 := contracts.SearchRequest{Query: "test", Engines: []string{"braveapi"}}
+
+	if Key(req1) == Key(req2) {
+		t.Error("different engines should produce different keys")
+	}
+}
+
+func TestKey_TimeRange(t *testing.T) {
+	req1 := contracts.SearchRequest{Query: "test"}
+	req2 := contracts.SearchRequest{Query: "test", TimeRange: strPtr("week")}
+
+	if Key(req1) == Key(req2) {
+		t.Error("with/without time_range should produce different keys")
+	}
+}
+
+func TestNew_NopWithoutAddress(t *testing.T) {
+	c := New(Config{}, nil)
+	if c.Enabled() {
+		t.Error("cache should be disabled when no address is configured")
+	}
+	if err := c.Close(); err != nil {
+		t.Errorf("Close on nop cache should not error: %v", err)
+	}
+}
+
+func strPtr(s string) *string { return &s }