Bootstrap copied BFF and local accounts with public Sendico contracts (#34)

backend/contracts/sendico/api/ledger_accounts/balance.yaml

@@ -0,0 +1,58 @@
+get:
+  tags: [Ledger Accounts]
+  summary: Get ledger account balance
+  description: |
+    Returns the canonical balance for the requested ledger account.
+
+    Without `as_of`, the response is the current materialized balance and includes
+    current metadata fields `version` and `lastUpdated`.
+
+    With `as_of`, the response is an event-time accounting balance reconstructed
+    from posted statement-entry deltas where `eventTime <= as_of`. Entries exactly
+    equal to `as_of` are included. This is not a historical snapshot of materialized
+    balance state and not "what the system knew" at that clock time. Late or
+    backdated postings with older `eventTime` can change historical responses for
+    the same `as_of`. Historical responses include `ledgerAccountRef`, `balance`,
+    and canonical UTC `asOf`, and omit `version` and `lastUpdated` because no
+    historical snapshot/version semantics are defined.
+
+    Historical balance is valid only if every account-affecting committed posting
+    writes matching statement entries in the same transaction as balance updates.
+  operationId: ledgerAccountBalanceGet
+  security:
+    - bearerAuth: []
+  parameters:
+    - $ref: ../parameters/organizations_ref.yaml#/components/parameters/OrganizationsRef
+    - $ref: ../parameters/ledger_account_ref.yaml#/components/parameters/LedgerAccountRef
+    - name: as_of
+      in: query
+      required: false
+      description: |
+        Optional RFC3339 event-time cutoff for historical balance reconstruction.
+        Timezone offsets are accepted and returned as canonical UTC `asOf`.
+      schema:
+        type: string
+        format: date-time
+      example: '2026-04-23T14:30:00+02:00'
+  responses:
+    '200':
+      description: Ledger account balance
+      content:
+        application/json:
+          schema:
+            allOf:
+              - $ref: ../response/response.yaml#/components/schemas/BaseResponse
+              - type: object
+                properties:
+                  data:
+                    $ref: ./response/balance.yaml#/components/schemas/LedgerAccountBalanceData
+    '400':
+      $ref: ../response/operation.yaml#/components/responses/BadRequest
+    '401':
+      $ref: ../response/operation.yaml#/components/responses/Unauthorized
+    '403':
+      $ref: ../response/operation.yaml#/components/responses/Forbidden
+    '404':
+      $ref: ../response/operation.yaml#/components/responses/NotFound
+    '500':
+      $ref: ../response/operation.yaml#/components/responses/InternalServerError