fix(model): return string from JSON column Valuers for pg simple protocol

With PrepareStmt disabled, PostgreSQL queries run over pgx's simple
protocol, which encodes every []byte parameter as a bytea hex literal
('\x...'). driver.Valuer implementations returning []byte from
json.Marshal therefore fail json-column writes with SQLSTATE 22P02
(reported on the channels UPDATE path via ChannelInfo).

Reproduced against a live PostgreSQL 16: []byte Valuer into a json
column fails under simple protocol, string succeeds; []byte into a
text column silently stores the hex literal (no such path exists in
the repo today — audited all Valuers, json.RawMessage fields, and raw
SQL call sites).

- ChannelInfo, Properties, TaskPrivateData, JSONValue Value() now
  return string; zero-value nil semantics unchanged. Task.Data
  (bare json.RawMessage) is unaffected — database/sql's default
  converter already passes it as expected.
- Their Scan() counterparts now accept both []byte and string via a
  shared jsonScanBytes helper: SQLite returns string for these columns
  once Value() emits string, and the old []byte-only assertions
  silently zeroed the field (caught by the model test suite).
- Add regression tests locking both contracts: json-column Valuers
  must return string (or nil for zero values), Scanners must accept
  []byte and string.

Verified end-to-end against PostgreSQL 16 with the real model types:
Channel create/update/read-back, Task json fields, PrefillGroup items.

model/task.go

@@ -87,7 +87,7 @@ type Properties struct {
 }
 
 func (m *Properties) Scan(val interface{}) error {
-	bytesValue, _ := val.([]byte)
+	bytesValue := jsonScanBytes(val)
 	if len(bytesValue) == 0 {
 		*m = Properties{}
 		return nil
@@ -99,7 +99,13 @@ func (m Properties) Value() (driver.Value, error) {
 	if m == (Properties{}) {
 		return nil, nil
 	}
-	return common.Marshal(m)
+	// 必须返回 string 而非 []byte:PG simple protocol 下 []byte 按 bytea 编码,
+	// 写 json 列会触发 SQLSTATE 22P02。
+	b, err := common.Marshal(m)
+	if err != nil {
+		return nil, err
+	}
+	return string(b), nil
 }
 
 type TaskPrivateData struct {
@@ -180,7 +186,7 @@ func GenerateTaskID() string {
 }
 
 func (p *TaskPrivateData) Scan(val interface{}) error {
-	bytesValue, _ := val.([]byte)
+	bytesValue := jsonScanBytes(val)
 	if len(bytesValue) == 0 {
 		return nil
 	}
@@ -191,7 +197,12 @@ func (p TaskPrivateData) Value() (driver.Value, error) {
 	if (p == TaskPrivateData{}) {
 		return nil, nil
 	}
-	return common.Marshal(p)
+	// 同 Properties.Value:string 避免 PG simple protocol 的 bytea 编码。
+	b, err := common.Marshal(p)
+	if err != nil {
+		return nil, err
+	}
+	return string(b), nil
 }
 
 // SyncTaskQueryParams 用于包含所有搜索条件的结构体，可以根据需求添加更多字段