fix: expand channels.group column length (#6017)

[cat0825]

⚠️ 提交说明 / PR Notice

Important

• 请提供人工撰写的简洁摘要，避免直接粘贴未经整理的 AI 输出。

📝 变更描述 / Description

将 channels.group 字段从 varchar(64) 扩展到 varchar(1024)，避免渠道关联多个/较长分组时保存失败，也避免升级后 AutoMigrate 把字段改回 64。

This PR was AI-assisted and manually reviewed before submission.

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix) - 请关联对应 Issue，避免将设计取舍、理解偏差或预期不一致直接归类为 bug
• ✨ 新功能 (New feature) - 重大特性建议先通过 Issue 沟通
• ⚡ 性能优化 / 重构 (Refactor)
• 📝 文档更新 (Documentation)

🔗 关联任务 / Related Issue

• Closes 渠道关联的分组数量较多会导致插入SQL报错 #6017

✅ 提交前检查项 / Checklist

• 人工确认: 我已亲自整理并撰写此描述，没有直接粘贴未经处理的 AI 输出。
• 非重复提交: 我已搜索现有的 Issues 与 PRs，确认不是重复提交。
• Bug fix 说明: 若此 PR 标记为 Bug fix，我已提交或关联对应 Issue，且不会将设计取舍、预期不一致或理解偏差直接归类为 bug。
• 变更理解: 我已理解这些更改的工作原理及可能影响。
• 范围聚焦: 本 PR 未包含任何与当前任务无关的代码改动。
• 本地验证: 已在本地运行并通过测试或手动验证，维护者可以据此复核结果。
• 安全合规: 代码中无敏感凭据，且符合项目代码规范。

📸 运行证明 / Proof of Work

Change is schema-tag only. Verified with:
git show HEAD:model/channel.go | rg 'Group.*varchar\(1024\)'

Summary by CodeRabbit

• Improvements
  • Increased the maximum supported length for channel group names.
  • Preserved the default group value for new channels.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 5e918b76-7f1d-4527-a982-ecb93550d506

📥 Commits

Reviewing files that changed from the base of the PR and between ad900bb and a33273b.

📒 Files selected for processing (1)

• model/channel.go

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Walkthrough

The Channel.Group GORM column mapping is widened from varchar(64) to varchar(1024), while its default remains 'default'.

Changes

Channel group column capacity

Layer / File(s)	Summary
Widen Channel.Group mapping model/channel.go	The GORM column type changes to varchar(1024) and retains the 'default' value.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Poem

I’m a bunny with a broader lane,
Groups can hop through without strain.
The default stays tucked in place,
While longer names now have space. 🐇

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: widening the channels.group column length.
Linked Issues check	✅ Passed	Changing the GORM column type to varchar(1024) addresses the 64-character limit and steers AutoMigrate to the wider schema.
Out of Scope Changes check	✅ Passed	The change is limited to widening channels.group, which is directly required by the linked issue.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[cat0825]

该分支当前可合并，PR Check 与 CodeRabbit 均已通过。PR 仍处于 draft，但 GitHub 拒绝由当前作者账号执行 ready for review（仓库权限不足）；烦请维护者协助转为 Ready。