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

---------

README.md

@@ -163,6 +163,7 @@ The repository also contains shared packages under `packages/` (for example `pac
 
    - Running the background workers? Also create `apps/jobs/.env` (e.g. `cp apps/api/.env apps/jobs/.env`); it needs `REDIS_URL`. The `packages/db/.env` used by the DB scripts is set up in step 6.
    - Optional for self-hosted Enterprise-only features (SSO, token-auth, no-tracking): set `LICENSE_KEY` in `apps/api/.env`.
+   - Optional upload size limits: set matching `UPLOAD_MAX_*_MB` values in **both** `apps/api/.env` and `apps/dashboard/.env` (see `apps/api/.env.example`). Defaults are 5 MB documents, 2 MB assignment files, 800 MB videos.
 
 6. Start local infrastructure for API (Postgres + Redis) and seed the DB:
 
@@ -249,7 +250,7 @@ cp .env.example .env   # copy env template, edit for your domain
 
 The script reads root `.env` via `docker compose --env-file .env` and auto-generates a secure `PRIVATE_SERVER_KEY` (and `BETTER_AUTH_SECRET`) when missing.
 
-See [`.env.example`](.env.example) for the full list of environment variables with required/optional grouping, and [`docker/docs/SELF_HOST.md`](docker/docs/SELF_HOST.md) for the complete Docker self-hosting guide.
+See [`.env.example`](.env.example) for the full list of environment variables with required/optional grouping, and [`docker/docs/SELF_HOST.md`](docker/docs/SELF_HOST.md) for the complete Docker self-hosting guide. To raise upload caps (documents, videos, assignment files), set the `UPLOAD_MAX_*_MB` vars in `.env` — see the [docs](https://docs.classroomio.com/docs/self-hosted/docker#optional--upload-file-size-limits).
 
 ## Publishing