feat(quotes): allow a per-transaction platform fee override on POST /quotes

openapi/components/schemas/quotes/PlatformFeeOverride.yaml

@@ -0,0 +1,48 @@
+type: object
+description: >-
+  Overrides the platform fee for this transaction. When present, it replaces
+  all platform fees that would otherwise apply to the transaction — no standing
+  fee config is required. Only supported when the quote's source currency is
+  USD today; the fixed fee must be denominated in the source currency.
+
+  Only honored on platform-authenticated requests to `POST /quotes`. Agent
+  tokens carry no fee-control permission, so this field must be omitted on
+  agent-authenticated requests such as `POST /agents/me/quotes`.
+required:
+  - platformFixedFee
+  - platformVariableFeeBps
+properties:
+  platformFixedFee:
+    type: object
+    description: Fixed fee charged for this transaction.
+    required:
+      - amount
+      - currency
+    properties:
+      amount:
+        type: integer
+        format: int64
+        minimum: 0
+        description: >-
+          Fee amount in the smallest unit of the fixed fee's `currency`
+          (e.g., cents for USD).
+        example: 50
+      currency:
+        type: string
+        description: >-
+          Three-letter currency code (ISO 4217) the fixed fee is denominated
+          in. Must equal the quote's source currency (USD today).
+        example: USD
+  platformVariableFeeBps:
+    type: integer
+    minimum: 0
+    maximum: 10000
+    description: >-
+      Variable fee in basis points (1 bps = 0.01%) to apply to the
+      transaction's source-currency amount.
+    example: 30
+example:
+  platformFixedFee:
+    amount: 50
+    currency: USD
+  platformVariableFeeBps: 30

openapi/components/schemas/quotes/QuoteRequest.yaml

@@ -67,6 +67,8 @@ properties:
     example: '12345'
   purposeOfPayment:
     $ref: ./PurposeOfPayment.yaml
+  platformFeeOverride:
+    $ref: ./PlatformFeeOverride.yaml
   scaFactor:
     $ref: ../sca/ScaFactor.yaml
     description: >-