feat(saas): multi-tenant isolation (Phase 1) + rate-limit/audit/org-scoped export (Phase 2)

Phase 1 (tenant isolation):
- g.org_id set on require_auth; assert_tenant()/current_org_id() choke-point helpers.
- Multi-org provisioning: POST /api/admin/users {new_org:true} (super_admin) creates a
  new org + its first admin; GET /api/admin/orgs (super_admin sees all, admin own).
- Fixed latent create_org double-id bug (dict id != store key).
- test_saas_tenant.py: org2 admin blocked from org1 group (403), can't list org1
  groups/users, sees only own org; super_admin sees all.

Phase 2 (hardening):
- Rate limit login (per-IP + per-username) + chat send (per-user) to protect LLM cost
  and slow brute force; services/rate_limit.py (in-memory + disk, no external deps).
- Audit log data/audit/audit.jsonl on org.create, user.promote_super_admin, analytics.export.
- CSV export now org-scoped (admin exports only own org).
All 8 backend suites pass.

backend/app/auth/users.py

@@ -31,10 +31,8 @@ class UserStore:
 
     # ── org ────────────────────────────────────────────────────────────
     def create_org(self, name: str, *, org_id: str | None = None) -> dict[str, Any]:
-        return self.orgs.create(
-            {"name": name, "id": org_id or new_id("org")},
-            key=org_id or new_id("org"),
-        )
+        oid = org_id or new_id("org")
+        return self.orgs.create({"name": name, "id": oid}, key=oid)
 
     def get_org(self, org_id: str) -> dict[str, Any]:
         return self.orgs.get(org_id)