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/question-types/src/exercise-types.ts

@@ -257,6 +257,8 @@ export interface ExerciseQuestionRendererProps {
   maxSubmissionItems?: number;
   disabled?: boolean;
   labels?: ExerciseQuestionLabels;
+  /** Platform-wide cap for exercise file uploads (megabytes). */
+  platformMaxFileSizeMb?: number;
   onAnswerChange?: (answer: AnswerData | null) => void;
   onQuestionChange?: (question: ExerciseQuestionModel) => void;
   onImageUpload?: ExerciseQuestionImageUploader;

packages/question-types/src/render-contract.ts

@@ -18,6 +18,8 @@ export interface ExerciseQuestionRenderContract {
   maxSubmissionItems?: number;
   disabled?: boolean;
   labels?: ExerciseQuestionLabels;
+  /** Platform-wide cap for exercise file uploads (megabytes). */
+  platformMaxFileSizeMb?: number;
   onImageUpload?: ExerciseQuestionImageUploader;
   onFileUpload?: ExerciseQuestionFileUploader;
   onVideoRecordingUpload?: ExerciseQuestionVideoRecordingUploader;
@@ -31,6 +33,8 @@ export interface ExerciseQuestionListRenderContract {
   maxSubmissionItems?: number;
   disabled?: boolean;
   labels?: ExerciseQuestionLabels;
+  /** Platform-wide cap for exercise file uploads (megabytes). */
+  platformMaxFileSizeMb?: number;
   onImageUpload?: ExerciseQuestionImageUploader;
   onFileUpload?: ExerciseQuestionFileUploader;
   onVideoRecordingUpload?: ExerciseQuestionVideoRecordingUploader;