Fix file-upload 404 on a brand-new chat

[pufit]

Problem

Attaching a file to a freshly-created chat fails. The browser console shows:

POST https://<host>/api/files/upload 404 (Not Found)

Root cause

It is not a missing route. POST /api/files/upload is registered and live (it's in the running app's OpenAPI; an unauthenticated POST returns 401, not 404). The 404 comes from inside the handler:

session = await deps.db.get_session(session_id)
if not session:
    raise HTTPException(status_code=404, detail="Session not found")

The lazy "new chat" flow mints a virtual session — a client-only randomUUID() (chatStore createSession) that is not persisted in the DB until the first message is sent. The code comment is explicit that this temp id "is never sent to the backend." But the file-upload path (ChatInput.addFiles → api.uploadFiles) fired immediately against that virtual id, so the backend — which had never seen it — returned 404 "Session not found". This hits every attach-to-new-chat, i.e. the most common upload moment.

Reproduced against a live server: uploading with a fake/virtual session_id → 404 "Session not found"; with a real persisted id → 200.

Fix

Materialize the virtual session before uploading, mirroring the existing first-message flow:

• Extract a shared ensureRealSession(running?) store action — it POSTs /api/sessions, adopts the server-minted id, carries the unsent draft across to the new id (so the composer doesn't blank when the id swaps), and is a no-op once the chat is already real.
• ChatInput.addFiles calls ensureRealSession() and uploads against the returned real id.
• sendMessage now reuses the same action instead of its own inline materialization (single source of truth; send path keeps is_running: true so the sidebar spinner is instant).

Frontend-only; no backend or DB change. The session must genuinely exist (uploads are keyed by session_id on disk and in uploaded_files), so materializing — not relaxing the guard — is the correct fix.

Test plan

• npm run build (tsc typecheck + Vite) passes
• eslint on both changed files: clean
• Live repro confirms fail/pass boundary (virtual id → 404, real id → 200)
• Manual: open a new chat, attach a file before sending any message → upload succeeds (200), session appears in the sidebar, message sends with the attachment

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