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>

backend/app/models/__init__.py

@@ -1,4 +1,5 @@
 from app.models.user import DuniterIdentity, Session
+from app.models.organization import Organization, OrgMember
 from app.models.document import Document, DocumentItem, ItemVersion
 from app.models.decision import Decision, DecisionStep
 from app.models.vote import VoteSession, Vote
@@ -9,6 +10,7 @@ from app.models.cache import BlockchainCache
 
 __all__ = [
     "DuniterIdentity", "Session",
+    "Organization", "OrgMember",
     "Document", "DocumentItem", "ItemVersion",
     "Decision", "DecisionStep",
     "VoteSession", "Vote",