fix: isolate concurrent generations and queues by conversation branch

[chenyme]

Summary

修复并发对话与分支切换时的生成、排队和分支选择问题。

• 支持不同会话以及同一会话不同分支并发生成。
• 按会话、分支路径和 run ID 隔离排队消息，避免消息发送至错误会话或分支。
• 编辑或重试消息后立即切换至新分支，并安全完成临时消息 ID 替换。
• 切换页面、会话或分支后，后台生成及其队列继续归属于原始分支。
• 为隐藏会话中的恢复流增加按需、低频终态查询，使后续队列能够自动派发。
• 修复并发任务完成时侧边栏消息数量被旧快照覆盖的问题。
• 停止生成仅影响当前可见分支，不会取消其他后台任务。

Fixes #524

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• pnpm --filter @deeix/web check
• git diff --check
• Frontend build not run; requires explicit elevation and was not requested.
• Manual browser verification was not run.

Screenshots, API examples, or logs

No API changes. No additional screenshots.

Configuration, migration, and compatibility notes

• No database migration or configuration changes.
• No backend or public API contract changes.
• Hidden resumed runs are queried only while a dependent queue exists outside the current conversation.
• Existing conversations and queued-message behavior remain compatible.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including authentication, authorization, provider routing, file processing, billing, and admin APIs where relevant.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.