Make RAG citations clickable for students (sources panel + linked sources)

[NoveliaYuki]

Problem

PR #430 added inline [N] citation markers to assistant answers, but they are plain text — a student cannot click them to reach the source. The underlying permalinks are relative and the /docs proxy requires a Bearer header, which a top-level browser navigation from the Open WebUI chat never carries. There is also no sources list rendered to the student.

Goal

A student chatting through Open WebUI should be able to click a citation and open the cited library item — shown under its real filename, with a download-original option — without logging in, and without OWI ever contacting the Library Manager directly (OWI → LAMB → Library).

Design (decided with the lead)

• Rendering: emit OWI's native top-level sources field in the stream so OWI renders a clickable, persisted citations panel and auto-links the inline [N] markers to it (no OWI changes; OWI strips markdown reference links so the panel/source.name="N" mechanism is used).
• Auth on click: because the click carries no LAMB identity, each link authorizes itself with an org-scoped HMAC signature — per-organization isolated (no cross-org access; revoke by rotating the secret) and no expiry (citations in old chats keep working).
• Target: the link opens a LAMB-served view page titled with the real filename, showing the item's markdown content, plus a "Download original" button for the actual uploaded file.

Scope

Backend only (LAMB completions + a new signed public route). The legacy authenticated /docs permalink is unchanged.

[NoveliaYuki]

Closed by #446 (merged into integration/kbserver-lamb-dev-main). Opt-in clickable citation links to cited sources, default off.