Show product usage types in credit ledger

Persist hotword request origin so every reservation lifecycle entry can be classified without mutating the immutable ledger.

docs/openapi.yaml

@@ -1042,6 +1042,10 @@ components:
         amount: { type: integer, format: int64 }
         balanceAfter: { type: integer, format: int64, minimum: 0 }
         reasonCode: { type: string }
+        usageType:
+          type: ["string", "null"]
+          enum: [polish, asr, ai, agent, hotword, null]
+          description: Product usage associated with this ledger operation
         createdAt: { type: string, format: date-time }
     AdminLedgerPage:
       type: object
@@ -1373,6 +1377,10 @@ components:
         maxOutputTokens: { type: integer, minimum: 1, maximum: 4096, default: 512 }
         temperature: { type: number, minimum: 0, maximum: 1, default: 0.2 }
         stream: { type: boolean, default: false }
+        requestSource:
+          type: ["string", "null"]
+          enum: [hotword, null]
+          description: Optional product entry point; hotword is accepted only for AI requests
     CreateGatewayGrantRequest:
       type: object
       additionalProperties: false