feat(10): Homework vision

[tpaulshippy]

Roadmap 10: Homework Vision

Implements #62 (voice portion split to #64).

What changed

• Backend: Image processing for worksheet capture, KaTeX math rendering pipeline, homework-mode prompt injection, vision model auto-upgrade
• Frontend: Camera integration for worksheet photos, KaTeX rendering in chat
• Capstone feature: photograph a worksheet and talk to a tutor that renders math
• Fixes since review: empty-string text child no longer trips the RN-web View check on send; web notification crash guarded

Evidence

Worksheet photographed in chat and sent as an image message:

pr52-vision-voice.mp4

[tpaulshippy]

Automated review — backwards-compat + tests

Backwards-compat: SAFE. Backend-only diff (+51/-0): bot.enable_vision default False, message.intent default 'chat', message.meta default dict. No serializer/URL changes, so old phones neither send nor see the new fields; stored defaults keep old rows valid.
Tests: NONE in this diff. No test files; PR body leans on evidence screenshots/video and the camera/KaTeX frontend described but not present in this diff.

MUST FIX

1. enable_vision is write-only — _upgrade_to_vision_model (chat.py) upgrades any image message regardless of the flag. Either gate on bot.enable_vision or remove the flag; as-is the per-bot vision switch in the product story does nothing.

SHOULD FIX

2. Vision-model picker AiModel.objects.filter(supported_input_modalities__contains=['image']) (JSONField default=list): no ordering guarantee on .first() and exclude(model_id=current) can bounce between two vision models across turns. Add explicit ordering and pin per-chat once chosen.
3. Silent fallback — no vision model → use_default_model with only a log warning; image gets sent to a text-only model path. Surface a user-facing error or disable image attach for that bot.

NICE TO HAVE

4. message.meta/intent have no serializer exposure in this diff — fine for compat, but confirm the follow-up exposes/validates them rather than letting clients free-write meta.