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/prefill_group.go

@@ -20,11 +20,13 @@ import (
 type JSONValue json.RawMessage
 
 // Value 实现 driver.Valuer 接口，用于数据库写入
+// 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。
 func (j JSONValue) Value() (driver.Value, error) {
 	if j == nil {
 		return nil, nil
 	}
-	return []byte(j), nil
+	return string(j), nil
 }
 
 // Scan 实现 sql.Scanner 接口，兼容不同驱动返回的类型