fix: add NEXT_API_DEBUG_MODE environment variable to MMDL

Resolves calendar creation issue where clicking save would fail with
'Cannot read properties of undefined (Reading 'toUpperCase')' error.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

roles/docker/templates/mmdl-env.j2

@@ -38,4 +38,5 @@ SESSION_VALIDITY_PERIOD=30
 API_URL=https://{{ subdomains.tasks }}
 DEBUG_MODE=false
 TEST_MODE=false
+NEXT_API_DEBUG_MODE=false
 SUBTASK_RECURSION_DEPTH=5