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

---------

apps/docs/content/docs/self-hosted/coolify.mdx

@@ -46,6 +46,9 @@ import { Callout } from 'fumadocs-ui/components/callout';
    storage instead? Point the same `OBJECT_STORAGE_*` variables at external **S3 / Cloudflare R2**
    and leave the `minio` profile off — see the [Cloudflare Setup](/docs/contributor-guides/cloudflare-setup) guide for R2.
 
+   **Upload size limits** (optional): set `UPLOAD_MAX_*_MB` in Coolify's env for both the `api` and
+   `dashboard` services (same values in each). See [Docker self-hosting → Upload file size limits](/docs/self-hosted/docker#optional--upload-file-size-limits).
+
 5. **Deploy.** Coolify pulls `classroomio/{api,dashboard,jobs}:${CIO_VERSION}`, starts Postgres + Redis, the API container runs database setup on first boot, then the dashboard and the **jobs worker** come up.
 
 ## Upgrades & rollback