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/channel.go

@@ -162,14 +162,19 @@ func ApplyChannelGroupFilter(query *gorm.DB, group string) *gorm.DB {
 }
 
 // Value implements driver.Valuer interface
+// 必须返回 string 而非 []byte:PG simple protocol 下 []byte 参数按 bytea
+// 编码,写 json 列会触发 SQLSTATE 22P02。
 func (c ChannelInfo) Value() (driver.Value, error) {
-	return common.Marshal(&c)
+	b, err := common.Marshal(&c)
+	if err != nil {
+		return nil, err
+	}
+	return string(b), nil
 }
 
 // Scan implements sql.Scanner interface
 func (c *ChannelInfo) Scan(value interface{}) error {
-	bytesValue, _ := value.([]byte)
-	return common.Unmarshal(bytesValue, c)
+	return common.Unmarshal(jsonScanBytes(value), c)
 }
 
 func (channel *Channel) GetKeys() []string {