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/core/src/services/agent/document.ts

@@ -1,12 +1,8 @@
 import { nanoid } from 'nanoid';
 import { AppError } from '@cio/utils/errors';
-import {
-  MAX_DOCUMENT_TEXT_LENGTH,
-  MAX_AGENT_DOCUMENT_SIZE,
-  DOCUMENT_REDIS_TTL,
-  SUPPORTED_DOCUMENT_TYPES
-} from '@cio/ai-assistant';
+import { MAX_DOCUMENT_TEXT_LENGTH, DOCUMENT_REDIS_TTL, SUPPORTED_DOCUMENT_TYPES } from '@cio/ai-assistant';
 import type { DocumentUploadResult } from '@cio/ai-assistant';
+import { getUploadLimits } from '../../config/upload-limits';
 import { agentDocumentKey } from '../../utils/redis-keys';
 import { trackAgentEvent, AgentEvent } from '../../utils/tinybird';
 import type { RedisClient } from '../../utils/redis/redis';
@@ -34,9 +30,11 @@ export async function parseAndStoreDocument(
     throw new AppError('Unsupported file type. Allowed: PDF, DOCX, PPTX', 'UNSUPPORTED_FILE_TYPE', 415);
   }
 
-  if (file.size > MAX_AGENT_DOCUMENT_SIZE) {
+  const maxAgentDocumentSize = getUploadLimits().agentDocumentBytes;
+
+  if (file.size > maxAgentDocumentSize) {
     throw new AppError(
-      `File too large. Maximum size is ${MAX_AGENT_DOCUMENT_SIZE / (1024 * 1024)}MB`,
+      `File too large. Maximum size is ${maxAgentDocumentSize / (1024 * 1024)}MB`,
       'FILE_TOO_LARGE',
       413
     );