GET /history?entity_id= (empty) returned 422 instead of an honest empty series

FIX 2026-09-06 (code landed, unverified on live): /history now accepts an empty entity_id (Query default ” instead of min_length=3) and returns {points:[], tracked:false} 200 for a blank id — an unresolved SPA series channel is an honest empty state, not a 422. The ‘tracked’ field (unmapped vs mapped-but-empty) was already present. Client hooks useHistory/get_entity_history already guard empty ids; this covers any other caller. — ORIGINAL tracker items: ‘GET /history?entity_id=&hours=X fires with empty id -> 422’ and ‘History can’t distinguish unmapped from mapped-but-empty’ (latter was already fixed via the tracked flag).