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/services/org_service.py

@@ -0,0 +1,60 @@
+"""Organization service: CRUD + membership helpers."""
+
+from __future__ import annotations
+
+import uuid
+from typing import Sequence
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.organization import OrgMember, Organization
+
+
+async def list_organizations(db: AsyncSession) -> Sequence[Organization]:
+    result = await db.execute(select(Organization).order_by(Organization.name))
+    return result.scalars().all()
+
+
+async def get_organization(db: AsyncSession, org_id: uuid.UUID) -> Organization | None:
+    return await db.get(Organization, org_id)
+
+
+async def get_organization_by_slug(db: AsyncSession, slug: str) -> Organization | None:
+    result = await db.execute(select(Organization).where(Organization.slug == slug))
+    return result.scalar_one_or_none()
+
+
+async def create_organization(db: AsyncSession, **kwargs) -> Organization:
+    org = Organization(**kwargs)
+    db.add(org)
+    await db.commit()
+    await db.refresh(org)
+    return org
+
+
+async def is_member(db: AsyncSession, org_id: uuid.UUID, identity_id: uuid.UUID) -> bool:
+    result = await db.execute(
+        select(OrgMember).where(
+            OrgMember.org_id == org_id,
+            OrgMember.identity_id == identity_id,
+        )
+    )
+    return result.scalar_one_or_none() is not None
+
+
+async def add_member(
+    db: AsyncSession, org_id: uuid.UUID, identity_id: uuid.UUID, role: str = "member"
+) -> OrgMember:
+    member = OrgMember(org_id=org_id, identity_id=identity_id, role=role)
+    db.add(member)
+    await db.commit()
+    await db.refresh(member)
+    return member
+
+
+async def list_members(db: AsyncSession, org_id: uuid.UUID) -> Sequence[OrgMember]:
+    result = await db.execute(
+        select(OrgMember).where(OrgMember.org_id == org_id).order_by(OrgMember.created_at)
+    )
+    return result.scalars().all()