Multi-tenancy : espaces de travail + fix auth reload (rate limiter OPTIONS)

- Modèles Organization + OrgMember, migration Alembic (SQLite compatible)
- organization_id nullable sur Document, Decision, Mandate, VotingProtocol
- Service, schéma, router /organizations + dependency get_active_org_id
- Seed : Duniter G1 + Axiom Team ; tout le contenu seed attaché à Duniter G1
- Backend : list/create filtrés par header X-Organization
- Frontend : store organizations, WorkspaceSelector réel, useApi injecte l'org
- Fix critique : rate_limiter exclut les requêtes OPTIONS (CORS preflight)
  → résout le bug "Failed to fetch /auth/me" au reload (429 sur preflight)

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

frontend/app/composables/useApi.ts

@@ -73,6 +73,7 @@ function isRetryable(status: number): boolean {
 export function useApi() {
   const config = useRuntimeConfig()
   const auth = useAuthStore()
+  const orgsStore = useOrganizationsStore()
 
   /**
    * Perform a typed fetch against the backend API.
@@ -94,6 +95,9 @@ export function useApi() {
     if (auth.token) {
       headers.Authorization = `Bearer ${auth.token}`
     }
+    if (orgsStore.activeSlug) {
+      headers['X-Organization'] = orgsStore.activeSlug
+    }
 
     const maxAttempts = noRetry ? 1 : MAX_RETRIES
     let lastError: any = null