fix(db): side_question checkpoint 入库前剥除 jsonb 不支持的 NUL 转义

HTTP/工具响应含裸 NUL 字节时，json.Marshal 将其编码为 \u0000 转义，
PostgreSQL jsonb 列拒绝并抛 SQLSTATE 22P05，导致 SaveSideSnapshot 的
INSERT 每秒失败数十次刷屏。新增 jsonbClean 在 marshal 后剥除真正的
\u0000 转义（按前置反斜杠奇偶判断，避免误伤字面 \\u0000），并订正
exploration.go 中 "json.Marshal already sanitizes" 的错误注释。

Closes #111, Closes #112

db/jsonb_clean_test.go

@@ -0,0 +1,44 @@
+package db
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func TestJsonbClean(t *testing.T) {
+	// A marshaled payload carrying a NUL byte (e.g. captured HTTP/tool output).
+	b, err := json.Marshal(map[string]string{"body": "ab\x00cd"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(b), `\u0000`) {
+		t.Fatalf("precondition: marshaled JSON should contain the NUL escape, got %s", b)
+	}
+
+	cleaned := jsonbClean(b)
+	if strings.Contains(string(cleaned), `\u0000`) {
+		t.Fatalf("jsonbClean left a NUL escape jsonb rejects: %s", cleaned)
+	}
+
+	// Result must stay valid JSON with the NUL simply dropped.
+	var out map[string]string
+	if err := json.Unmarshal(cleaned, &out); err != nil {
+		t.Fatalf("cleaned bytes are not valid JSON: %v (%s)", err, cleaned)
+	}
+	if out["body"] != "abcd" {
+		t.Fatalf("expected NUL stripped to \"abcd\", got %q", out["body"])
+	}
+
+	// A NUL escape typed literally in source text (doubled backslash) is preserved.
+	lit := []byte(`{"body":"\\u0000"}`)
+	if got := jsonbClean(lit); string(got) != string(lit) {
+		t.Fatalf("literal \\\\u0000 must be untouched, got %s", got)
+	}
+
+	// No NUL escape → returned unchanged.
+	plain := []byte(`{"body":"hello"}`)
+	if got := jsonbClean(plain); string(got) != string(plain) {
+		t.Fatalf("plain JSON must be untouched, got %s", got)
+	}
+}