## ADDED Requirements
### Requirement: Request logs persist requested, actual, and billable service tiers separately
For Responses proxy traffic, the system MUST persist the operator-requested tier, the upstream-reported actual tier when available, and the effective billable tier used for pricing as separate request-log fields.

#### Scenario: Upstream reports a downgraded actual tier
- **WHEN** a client sends a Responses request with `service_tier: "priority"`
- **AND** the upstream response later reports `service_tier: "default"`
- **THEN** the persisted request log entry records `requested_service_tier = "priority"`
- **AND** the persisted request log entry records `actual_service_tier = "default"`
- **AND** the persisted request log entry records billable `service_tier = "default"`

#### Scenario: Upstream omits the actual tier
- **WHEN** a client sends a Responses request with `service_tier: "priority"`
- **AND** the upstream response omits `service_tier`
- **THEN** the persisted request log entry records `requested_service_tier = "priority"`
- **AND** the persisted request log entry records `actual_service_tier = null`
- **AND** the persisted request log entry records billable `service_tier = "priority"`
