feat: configurable upload file size limits via .env (#695)

* feat: make upload file size limits configurable via .env

Add UPLOAD_MAX_*_MB environment variables for documents, images, videos,
exercise assignment files, AI assistant documents, landing page images,
and media thumbnails. API presign routes validate file size server-side;
dashboard UI reads the same limits from layout data.

* chore: wire upload limit env vars through Docker compose and docs

Pass UPLOAD_MAX_*_MB from the root .env into api and dashboard containers
in both compose files, document them in .env.example and self-hosting guides.

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

* docs: cross-link upload limits in README, Railway, Coolify, and Cloudflare guides

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

* fix: address PR review feedback on upload limits

- Restore 5 MB default for thumbnail uploads (edit-asset-dialog regression)
- Add getResolvedUploadLimits() fallback for Storybook/tests
- Cache core getUploadLimits() like the dashboard helper
- Document presign fileSize as advisory; allow zero-byte fileSize in schema
- Only send fileSize to presign when greater than zero

* docs: remove upload limits from Cloudflare setup guide

---------

.env.example

@@ -124,6 +124,19 @@ ANTHROPIC_API_KEY=
 # Enterprise license (SSO, token-auth, no-tracking)
 LICENSE_KEY=
 
+# ──────────────────────────────────────────────
+# Optional — Upload file size limits (megabytes)
+# ──────────────────────────────────────────────
+# Unset = built-in defaults. Set on both api and dashboard (compose passes these through).
+# Also raise any reverse-proxy body-size limit (nginx client_max_body_size, etc.) if you go high.
+# UPLOAD_MAX_DOCUMENT_MB=5          # Course lesson documents (PDF, DOCX, DOC)
+# UPLOAD_MAX_IMAGE_MB=5               # Avatars, media library, editor images
+# UPLOAD_MAX_VIDEO_MB=800             # Lesson video uploads
+# UPLOAD_MAX_EXERCISE_FILE_MB=2       # Student assignment file-upload questions
+# UPLOAD_MAX_AGENT_DOCUMENT_MB=5      # AI assistant document uploads
+# UPLOAD_MAX_LANDING_IMAGE_MB=0.5     # Org landing page cover images
+# UPLOAD_MAX_THUMBNAIL_MB=5           # Media thumbnail uploads
+
 # Custom branding (meta tags)
 # PUBLIC_APP_TITLE=My Learning Platform
 # PUBLIC_APP_DESCRIPTION=A flexible LMS