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>

db/migrations/018-ideas.sql

@@ -0,0 +1,15 @@
+-- Ideation feature: shared ideas table for cross-tenant idea submissions
+CREATE TABLE IF NOT EXISTS shared.ideas (
+  id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+  org_id UUID NOT NULL REFERENCES shared.organizations(id) ON DELETE CASCADE,
+  user_id UUID NOT NULL REFERENCES shared.users(id) ON DELETE CASCADE,
+  title VARCHAR(255) NOT NULL,
+  description TEXT,
+  status VARCHAR(20) NOT NULL DEFAULT 'new',
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_ideas_org_id ON shared.ideas(org_id);
+CREATE INDEX IF NOT EXISTS idx_ideas_status ON shared.ideas(status);
+CREATE INDEX IF NOT EXISTS idx_ideas_created_at ON shared.ideas(created_at DESC);