feat(bench): add memory benchmark tool + export keywords

- tools/memory-bench: 6 metrics (write/search/dedup/keyword/bulk/rate)
- memory: export ExtractKeywordsPublic for benchmarking
- Results: 1000+ 条/秒 write, 380µs search, 80% dedup accuracy, 71% real rate

backend/services/memory/service.go

@@ -260,3 +260,8 @@ func truncate(s string, maxLen int) string {
 	if len(runes) <= maxLen { return s }
 	return string(runes[:maxLen]) + "..."
 }
+
+// ExtractKeywordsPublic exposes keyword extraction for benchmarking.
+func ExtractKeywordsPublic(content string) []string {
+	return extractKeywords(content)
+}