feat: add ideation feature with per-tenant toggle

Adds idea submission capability gated by a per-tenant feature flag.
Super admins can enable/disable ideation for specific tenants via the
admin tenant detail drawer. Users see a lightbulb icon in the header
when enabled, opening a modal to submit ideas (title + description).
Ideas are stored in shared schema for cross-tenant backlog querying.

- Database: shared.ideas table (018-ideas.sql migration)
- Backend: Ideas NestJS module (entity, service, controller)
- Admin API: GET /admin/ideas, PUT /admin/ideas/:id/status,
  PUT /admin/organizations/:id/settings
- Frontend: IdeaModal component, lightbulb ActionIcon in header
- Admin UI: Feature Toggles card with ideation Switch in drawer

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/modules/ideas/dto/create-idea.dto.ts

@@ -0,0 +1,12 @@
+import { IsString, IsNotEmpty, IsOptional, MaxLength } from 'class-validator';
+
+export class CreateIdeaDto {
+  @IsString()
+  @IsNotEmpty()
+  @MaxLength(255)
+  title: string;
+
+  @IsString()
+  @IsOptional()
+  description?: string;
+}