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/api/admin_routes.py

@@ -16,6 +16,43 @@ def _store():
     return current_app.extensions["user_store"]
 
 
+def _log_audit(action: str, subject: str, *, detail: dict | None = None) -> None:
+    """Append a line to the audit log (sensitive platform actions)."""
+    import json
+    import time
+    from ..config import Config
+
+    entry = {
+        "ts": time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime()),
+        "actor": (current_user() or {}).get("username") or (current_user() or {}).get("id"),
+        "action": action,
+        "subject": subject,
+        "detail": detail or {},
+    }
+    try:
+        log_dir = Config.DATA_DIR / "audit"
+        log_dir.mkdir(parents=True, exist_ok=True)
+        with open(log_dir / "audit.jsonl", "a", encoding="utf-8") as fh:
+            fh.write(json.dumps(entry, ensure_ascii=False) + "\n")
+    except Exception:
+        # Audit logging must never break the request.
+        pass
+
+
+@admin_bp.get("/orgs")
+@require_auth
+@require_roles("admin")
+def list_orgs():
+    """Platform view: all organizations (super_admin). Admin sees only their own."""
+    actor = current_user()
+    if actor.get("role") == "super_admin":
+        orgs = list(_store().orgs.all())
+        return jsonify({"orgs": orgs})
+    # plain admin: only their own org
+    org = _store().orgs.get_or_none(actor.get("org_id"))
+    return jsonify({"orgs": [org] if org else []})
+
+
 @admin_bp.post("/users")
 @require_auth
 @require_roles("admin")
@@ -36,13 +73,26 @@ def create_user():
     actor_role = current_user().get("role")
     if role in ("admin", "super_admin") and actor_role != "super_admin":
         raise ApiError("only super_admin can grant admin roles", 403)
+
+    # Multi-tenant: provisioning a brand-new org (super_admin only). The created user
+    # becomes that org's first admin (tenant owner). Everything is isolated per org.
+    new_org = bool(data.get("new_org"))
+    if new_org:
+        if actor_role != "super_admin":
+            raise ApiError("only super_admin can create a new organization", 403)
+        if role != "admin":
+            raise ApiError("new-org first user must be role=admin", 400)
+        org = _store().create_org(name or username)
+        org_id = org["id"]
+        _log_audit("org.create", org_id, detail={"name": name or username})
+
     try:
         user = _store().create_user(
             org_id=org_id, username=username, password=password, name=name, role=role
         )
     except AuthError as exc:
         raise ApiError(str(exc))
-    return jsonify({"user": _store().public_user(user)}), 201
+    return jsonify({"user": _store().public_user(user), "org_id": org_id}), 201
 
 
 @admin_bp.get("/users")
@@ -76,6 +126,8 @@ def update_user(username: str):
         if actor.get("role") != "super_admin":
             raise ApiError("only super_admin can change roles")
         _store().set_role(username, role)
+        if role == "super_admin":
+            _log_audit("user.promote_super_admin", username)
 
     if "active" in data:
         if actor.get("role") != "super_admin":