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/dashboard/src/lib/features/media/components/edit-asset-dialog.svelte

@@ -12,6 +12,7 @@
   import { snackbar } from '$features/ui/snackbar/store';
   import { t } from '$lib/utils/functions/translations';
   import { uploadImage } from '$lib/utils/services/upload';
+  import { getResolvedUploadLimits } from '$lib/utils/config/upload-limits-context';
 
   interface Props {
     open?: boolean;
@@ -22,7 +23,8 @@
 
   type AssetStatusValue = 'active' | 'archived';
 
-  const THUMBNAIL_MAX_SIZE = 5 * FileDropZone.MEGABYTE;
+  const uploadLimits = getResolvedUploadLimits();
+  const thumbnailMaxSize = uploadLimits.thumbnailBytes;
 
   let { open = $bindable(false), asset = null, isSaving = false, onSave = async () => {} }: Props = $props();
 
@@ -126,7 +128,7 @@
               accept={FileDropZone.ACCEPT_IMAGE}
               maxFiles={1}
               fileCount={0}
-              maxFileSize={THUMBNAIL_MAX_SIZE}
+              maxFileSize={thumbnailMaxSize}
               onUpload={handleThumbnailFilesUpload}
               onFileRejected={handleThumbnailRejected}
             >

apps/dashboard/src/lib/features/media/components/manage-thumbnails-dialog.svelte

@@ -14,6 +14,7 @@
   import { t } from '$lib/utils/functions/translations';
   import { uploadImage } from '$lib/utils/services/upload';
   import { classroomio } from '$lib/utils/services/api';
+  import { getResolvedUploadLimits } from '$lib/utils/config/upload-limits-context';
 
   function buildHlsUrl(asset: OrganizationAsset): string | null {
     if (!asset.hlsManifestKey) return null;
@@ -32,7 +33,8 @@
     onThumbnailSaved?: (asset: OrganizationAsset) => void;
   }
 
-  const THUMBNAIL_MAX_SIZE = 1 * FileDropZone.MEGABYTE;
+  const uploadLimits = getResolvedUploadLimits();
+  const thumbnailMaxSize = uploadLimits.thumbnailBytes;
   const REGEN_POLL_INTERVAL_MS = 2000;
   const REGEN_POLL_TIMEOUT_MS = 60_000;
   const SLOT_KEYS = ['start', 'middle', 'end'] as const;
@@ -289,7 +291,7 @@
                 accept={FileDropZone.ACCEPT_IMAGE}
                 maxFiles={1}
                 fileCount={0}
-                maxFileSize={THUMBNAIL_MAX_SIZE}
+                maxFileSize={thumbnailMaxSize}
                 onUpload={handleCustomUpload}
                 onFileRejected={handleUploadRejected}
               >