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 <[email protected]>

* docs: cross-link upload limits in README, Railway, Coolify, and Cloudflare guides

Co-authored-by: Rotimi Best <[email protected]>

* 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/course/components/exercise/edit-mode.svelte

@@ -44,8 +44,11 @@
   import SectionEditor from './section-editor.svelte';
   import type { Question } from '$features/course/types';
   import type { ExerciseEditorErrors } from './store';
+  import { getResolvedUploadLimits } from '$lib/utils/config/upload-limits-context';
 
   const flipDurationMs = 200;
+  const uploadLimits = getResolvedUploadLimits();
+  const platformMaxFileSizeMb = uploadLimits.exerciseFileMb;
   const initialQuestionsLength = $questionnaire.questions.length;
   type QuestionDndEvent = CustomEvent<{ items: Question[] }>;
 
@@ -390,6 +393,7 @@
           mode: 'edit',
           question: toExerciseQuestionModel(question),
           labels: questionLabels,
+          platformMaxFileSizeMb,
           onImageUpload: uploadImage
         }}
         onQuestionChange={(nextQuestion) => onSharedQuestionChange(question.id, nextQuestion)}