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

---------

docker/docs/SELF_HOST.md

@@ -88,6 +88,7 @@ Key points:
 - **Auto-configured:** All `MINIO_*` / `OBJECT_STORAGE_*` vars (by the startup script, with randomized MinIO credentials).
 - **Email (effectively required):** SMTP (or Zoho). See [Email](#email) — without it, signup verification, password reset, and invites do not send.
 - **Optional:** Google OAuth, Unsplash, `LICENSE_KEY` (enterprise).
+- **Optional — upload limits:** `UPLOAD_MAX_*_MB` vars (documents, images, videos, assignment files, etc.). See [`.env.example`](../../.env.example). Passed through to both `api` and `dashboard` containers; unset values use built-in defaults. Raise your reverse-proxy body-size limit too if you increase these.
 
 `PRIVATE_SERVER_KEY` must be the same value in both API and dashboard — the script ensures this.