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

---------

apps/api/.env.example

@@ -89,6 +89,15 @@ MOONSHOT_API_KEY=""
 # AGENT_MAX_FETCHES_PER_CONVERSATION=15
 # AGENT_MAX_FETCHES_PER_ORG_PER_DAY=500
 
+# Upload file size limits (megabytes). Set these on both API and dashboard for self-hosted installs.
+# UPLOAD_MAX_DOCUMENT_MB=5
+# UPLOAD_MAX_IMAGE_MB=5
+# UPLOAD_MAX_VIDEO_MB=800
+# UPLOAD_MAX_EXERCISE_FILE_MB=2
+# UPLOAD_MAX_AGENT_DOCUMENT_MB=5
+# UPLOAD_MAX_LANDING_IMAGE_MB=0.5
+# UPLOAD_MAX_THUMBNAIL_MB=5
+
 # Tinybird event tracking for AI observability (optional — silently skipped when not set)
 # TINYBIRD_TOKEN=""
 # TINYBIRD_BASE_URL="https://api.tinybird.co"