Fix Helm template error: update _helpers.tpl to use app instead of api

- Changed .Values.api.env.databaseUrl to .Values.app.env.databaseUrl
- This aligns with the unified architecture where api and frontend are combined into a single app
- Chart now passes helm lint successfully

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

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

helm/warehouse13/templates/_helpers.tpl

@@ -63,8 +63,8 @@ Create the name of the service account to use
 PostgreSQL connection string
 */}}
 {{- define "warehouse13.postgresUrl" -}}
-{{- if .Values.api.env.databaseUrl }}
-{{- .Values.api.env.databaseUrl }}
+{{- if .Values.app.env.databaseUrl }}
+{{- .Values.app.env.databaseUrl }}
 {{- else }}
 {{- printf "postgresql://%s:%s@warehouse13-postgres:%d/%s" .Values.postgres.auth.username .Values.postgres.auth.password (.Values.postgres.service.port | int) .Values.postgres.auth.database }}
 {{- end }}