Add R2 image storage, upload endpoint, and CDN support

- Backend: R2StorageService, upload controller (POST /api/upload)
- Frontend: CDN url helper, NEXT_PUBLIC_CDN_URL build arg
- Deploy: pass R2 secrets from Woodpecker CI to containers via .env
- Docs: update CLAUDE.md with CDN and upload conventions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/Dockerfile

@@ -7,6 +7,7 @@ FROM node:20-alpine AS builder
 WORKDIR /app
 ARG NEXT_PUBLIC_POSTHOG_KEY
 ARG NEXT_PUBLIC_POSTHOG_HOST
+ARG NEXT_PUBLIC_CDN_URL
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 RUN npm run build