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/api/src/routes/agent/agent.ts

@@ -6,6 +6,7 @@ import { orgAdminMiddleware } from '@api/middlewares/org-admin';
 import { authOrApiKeyMiddleware } from '@api/middlewares/auth-or-api-key';
 import { b64EnvelopeRewrite } from '@api/middlewares/b64-envelope';
 import { handleError, AppError } from '@api/utils/errors';
+import { MAX_AGENT_DOCUMENT_SIZE } from '@api/constants/upload';
 import { zValidator } from '@hono/zod-validator';
 import { streamText, stepCountIs, convertToModelMessages } from 'ai';
 import {
@@ -171,7 +172,7 @@ const agentCoreRouter = new Hono()
     } catch (error) {
       if (error instanceof AppError) {
         if (error.statusCode === 413) {
-          return c.json({ success: false, error: 'file_too_large', maxSize: 5242880 }, 413);
+          return c.json({ success: false, error: 'file_too_large', maxSize: MAX_AGENT_DOCUMENT_SIZE }, 413);
         }
 
         if (error.statusCode === 415) {