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

---------

packages/ui/src/custom/exercise-question/renderers/file-upload/take.svelte

@@ -11,6 +11,7 @@
     answer = null,
     disabled = false,
     labels,
+    platformMaxFileSizeMb,
     onAnswerChange = () => {},
     onFileUpload
   }: ExerciseQuestionRendererProps = $props();
@@ -86,7 +87,7 @@
 
     if (!selectedFile) return;
     if (!isFileTypeAllowed(selectedFile, acceptedTypes)) return;
-    if (!isFileSizeAllowed(selectedFile, maxSizeMb)) return;
+    if (!isFileSizeAllowed(selectedFile, maxSizeMb, platformMaxFileSizeMb)) return;
 
     if (!onFileUpload) return;