Classes: content upload on the class page with automatic class association

[shreekardittakavi]

Spec: Doc Task Magnitude — "Within the class page we have 'Content Upload'. This content upload allows..." (the doc sentence is unfinished; scope confirmed from surrounding context: course materials uploaded against a class become context for that class).

Current state

classes/[runId]/page.tsx shows a read-only Linked notes card. Notes get a classId only when created elsewhere and associated after the fact. There is no upload affordance on the class page itself.

Scope

• Upload control on the class detail page accepting the same types as notes/upload (png, jpeg, webp, gif, pdf).
• Uploaded material is parsed through the existing note-generation pipeline and auto-associated to the class via note.classId — no second extraction path.
• Multi-file upload with per-file progress and per-file failure, so one bad PDF does not sink the batch.
• Uploaded content is embedded with class metadata so it joins the class's retrievable context.
• Distinguish uploaded course material (lecture slides, problem sets, textbook chapters) from student-authored notes in the class view.

Depends on

• Syllabus/class embedding with metadata, for the context half

Acceptance criteria

• A user can upload course material from the class page and it appears linked to that class
• Uploaded material is retrievable as context when generating quizzes or flashcards for that class
• Per-file progress and failure states (AGENTS.md §1)
• Uploaded material is visually distinct from authored notes