feat: initialize project backend structure and document system specifications across all modules

docs/specs/onetargeting/audience_upload.md

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+---
+title: "Audience Upload"
+type: OpenSpec
+status: Draft
+domain: OneTargeting
+---
+
+# Audience Upload
+
+## Requirement Definition
+Provides a module for marketers to manage audience uploads, enabling them to upload customer data for targeted marketing campaigns via file uploads.
+
+## Process Flow
+
+### Pre-Conditions
+- Users must have access rights in the "Audience Upload" module.
+- Depending on access rights, users can view, add, edit, or manually approve.
+
+## Business Rules
+- Users can upload customer data for targeted marketing campaigns via file upload (CSV, Excel spreadsheets).
+- The system allows users to map data from source files to required platform fields.
+- The system validates the uploaded data to ensure all required fields are present and in the correct format.
+
+### Data Schema
+
+| Field Name | Description | Data Type | Constraints / Rules |
+|---|---|---|---|
+| Audience Name | The name of the audience | String (50) | Required. Must be unique in the system. |
+| Description | Audience description | String (200) | Optional. |
+| File ID | Audience identifier | String (10) | Required. Internal system reference. |
+| File Size | The file size of the upload | Numeric | System generated. |
+| Upload File | The uploaded file data | File/Binary | Required. Allowed extensions: CSV, XLS, XLSX. Max size: <= 50 MB. |