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/main.go

@@ -27,6 +27,19 @@ var commonFalseVal string
 var logKeyCol string
 var logGroupCol string
 
+// jsonScanBytes 归一化 json 列的驱动返回值:不同驱动/协议模式下同一列可能
+// 以 []byte 或 string 返回,静默丢弃 string 会导致字段被清零而不报错。
+func jsonScanBytes(value interface{}) []byte {
+	switch v := value.(type) {
+	case []byte:
+		return v
+	case string:
+		return []byte(v)
+	default:
+		return nil
+	}
+}
+
 func initCol() {
 	// init common column names
 	if common.UsingMainDatabase(common.DatabaseTypePostgreSQL) {