feat(circles): create_circle + add_member_to_circle (v0.7.0)

- acccreatefamily endpoint creates a new circle (returns numeric ID)
- accinvite endpoint invites new users by email (familyId, identifier, role, firstname)
- fw_client now detects a00.ex errors (was only checking a00.un before)
- New modules/circles.py with FamilyRoleTypeEnum constants
- SPEC.md updated with acccreatefamily, accinvite, accupdatefamily docs
- Note: circle deletion not supported by FW API (metadelete → "delete not supported")
- Note: accinvite only works for new (non-existing) FW accounts

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

src/mcp_familywall/fw_client.py

@@ -185,12 +185,17 @@ class FamilyWallClient:
             msg = f"API error from {endpoint!r}: {error_data}"
             raise FamilyWallError(msg, response_data=body)
 
-        # Some endpoints (e.g. taskupdate2) return per-call errors nested under
-        # a00.un.un instead of top-level — detect and surface them.
+        # Some endpoints return per-call errors nested under a00.un.un or a00.ex.ex
+        # instead of top-level — detect and surface them.
         a00 = body.get("a00", {})
-        if isinstance(a00, dict) and "un" in a00:
-            nested = a00["un"]
-            msg = f"API error from {endpoint!r}: {nested}"
-            raise FamilyWallError(msg, response_data=body)
+        if isinstance(a00, dict):
+            if "un" in a00:
+                nested = a00["un"]
+                msg = f"API error from {endpoint!r}: {nested}"
+                raise FamilyWallError(msg, response_data=body)
+            if "ex" in a00:
+                nested = a00["ex"]
+                msg = f"API error from {endpoint!r}: {nested}"
+                raise FamilyWallError(msg, response_data=body)
 
         return body