feat: add public answer sharing

[LittleChenLiya]

问题原因

当前聊天回答只能在登录后的原会话中查看，用户无法把某个问答片段生成可公开访问的只读链接。

修改内容

• 新增分享快照接口：登录用户可从有权限的 thread 中按消息 ID 创建公开分享，公开读取仅开放 GET /api/shares/{share_id}。
• 在助手回答操作区增加分享按钮，生成并复制 /share/{share_id} 链接。
• 新增公开只读分享页，免登录展示已分享的问答快照。

关联 issue

Closes #3288

Problem Cause

Chat answers can currently only be viewed from the authenticated source conversation, so users cannot create a public read-only link for a selected Q&A snippet.

Changes

• Add share snapshot APIs: authenticated users can create a public share from an authorized thread by message ID, while anonymous access is limited to GET /api/shares/{share_id}.
• Add a share action to assistant answer controls that creates and copies a /share/{share_id} link.
• Add a public read-only share page that displays the shared Q&A snapshot without login.

Related Issue

Closes #3288

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a “public share” feature for conversation threads, including backend endpoints to create/read immutable share snapshots and a frontend UI flow to generate and copy share links, plus tests and auth middleware updates to allow anonymous share reads.

Changes:

• Backend: introduce /api/shares router (create share from owned thread; public read by share id) and update auth middleware to allow anonymous GETs for share reads.
• Frontend: add createThreadShare API + types, add share button in MessageList, and add a new /share/[share_id] page for public viewing.
• Tests: add backend router/auth middleware tests and a frontend unit test for the share create API.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
frontend/tests/unit/core/threads/api.test.ts	Adds a unit test validating createThreadShare request payload.
frontend/src/core/threads/types.ts	Introduces share response types used by the frontend.
frontend/src/core/threads/api.ts	Adds createThreadShare API wrapper for creating a share.
frontend/src/components/workspace/messages/message-list.tsx	Adds share action UI per assistant turn and supporting logic.
frontend/src/app/share/[share_id]/page.tsx	New public share viewer page that loads and displays shared messages.
backend/tests/test_shares_router.py	Adds coverage for share creation, validation, and public read behavior.
backend/tests/test_auth_middleware.py	Adds coverage for anonymous share reads vs protected share creation.
backend/app/gateway/routers/shares.py	Implements share create/read endpoints and snapshot persistence.
backend/app/gateway/routers/init.py	Exposes the new shares router module.
backend/app/gateway/auth_middleware.py	Adds method+path-based public-route detection for share reads.
backend/app/gateway/app.py	Mounts the new shares router into the gateway app.

[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 7 comments.

[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

[WillemJiang]

@LittleChenLiya, we cannot publicly share the thread without the owner's permission. How do we store this permission in the system?

[LittleChenLiya]

@WillemJiang

现在公开分享权限会作为内部分享元数据持久化，包括 source_thread_id、created_by_user_id、选中的 message_ids、visibility 和 granted_at。公开读取接口仍只返回不可变快照内容，不会通过 GET /api/shares/{share_id} 暴露这些授权元数据。

我也收紧了创建分享的权限边界：公开分享必须来自明确归当前用户所有的 thread，不再接受 legacy/null-owner thread 的宽松访问路径。同时新增 DELETE /api/shares/{share_id}，创建者可以撤销公开链接；撤销后的链接返回 404，但内部会保留撤销记录。

验证：

• uv run python -m pytest tests/test_shares_router.py tests/test_auth_middleware.py -v
• uvx ruff check app/gateway/routers/shares.py tests/test_shares_router.py tests/test_auth_middleware.py
• uvx ruff format --check app/gateway/routers/shares.py tests/test_shares_router.py tests/test_auth_middleware.py

---

Addressed in 903bcf9.

Public share permission is now persisted as internal share metadata, including source_thread_id, created_by_user_id, selected message_ids, visibility, and granted_at. Public reads still return only the immutable snapshot fields, so this authorization metadata is not exposed by GET /api/shares/{share_id}.

I also tightened the creation boundary: public shares must come from a thread explicitly owned by the current user, instead of accepting the legacy/null-owner access path. I added DELETE /api/shares/{share_id} so the creator can revoke a public link; revoked links return 404 while keeping the revocation record internally.

Validation:

• uv run python -m pytest tests/test_shares_router.py tests/test_auth_middleware.py -v
• uvx ruff check app/gateway/routers/shares.py tests/test_shares_router.py tests/test_auth_middleware.py
• uvx ruff format --check app/gateway/routers/shares.py tests/test_shares_router.py tests/test_auth_middleware.py