Mark MCP chat posts with a fixed connected-app metadata key #336

Parent

#328. Related: #230, #344

What to build

post_chat_message posts in a heading chat as the document owner. Nothing in the stored row says a connected app wrote it. Past posts can never be marked, because no record ties a chat row to MCP.

After this change, every MCP chat post stores metadata.via = 'connected_app'. read_chat_thread reads the key back and tags those posts. So a connected app can tell its own posts from the owner's. The visible label in the pad is #344.

Acceptance criteria

• A message posted with post_chat_message has metadata equal to {"via":"connected_app"} in public.messages.
• The value is one fixed constant, CONNECTED_APP_VIA, in apps/hocuspocus.server/src/modules/mcp/types.ts. No client_id or app name is stored.
• read_chat_thread shows via connected app in the head line of such a post.
• From metadata, readThread selects only named paths, never the whole object.
• Composer messages have no via key. The pad renders an MCP post as before.
• After pin, unpin and a reply, metadata.via is still there.
• apps/hocuspocus.server/API.md §MCP connector, docs/mcp/reference.md and the MCP entry under ## [Unreleased] in apps/hocuspocus.server/CHANGELOG.md each state the key in one line.

Blocked by

None — can start now.

Agent brief

Type: AFK.

1. postMessage in apps/hocuspocus.server/src/modules/mcp/infra/chatStore.ts adds metadata: { via: CONNECTED_APP_VIA }. The store owns the constant, so no tool can pass another value.
2. readThread adds the path via:metadata->>via. ChatMessage gets a viaConnectedApp flag. renderChatMessage adds the tag inside frameChatText.
3. No migration and no apps/webapp/src/types/supabase.ts change: messages.metadata is an existing jsonb column. No insert trigger rewrites NEW.metadata.

Rules: apps/hocuspocus.server/CLAUDE.md §MCP Connector. AGENTS.md §Test Policy: no new test.

Verify: typecheck, bun test src/modules/mcp, lint and format. On the local stack, post with post_chat_message and check the row in Supabase Studio. Pin, unpin and reply, then call read_chat_thread.

Out of scope

• The visible label in the pad — Show a "via connected app" label on chat posts from a connected app #344.
• Labels in notifications, push, email, bookmarks or the pinned list.
• Any SQL change.