Api Limit Policies Update

Live counter state for ONE policy in its current window.

Nested on LimitPolicyListSerializer / LimitPolicyDetailSerializer. Produced by one Redis HGET limit:fw:{meter_id} counter — the detail endpoint knows the exact meter_definition_id so it can skip CH and hit Redis directly. The list endpoint batches the HGETs into a single pipeline RTT via LimitPoliciesView's prefetch hook so every row in a page resolves without N round-trips.

Same field names as the per-row live fields on LimitPolicyStateRowSerializer so the FE reuses one gauge component across both shapes.

Fail-open: Redis errors, unresolvable period, unsupported algorithm (token_bucket is stubbed in v1) all collapse to zeros + null bounds rather than 5xx-ing the config read.

GenericTag rows assigned to this policy. Managed via /api/tag-assignments/limit_policies/... — read-only here.