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

docs/specs/reward_campaign/transaction_category.md

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+---
+title: "Transaction Category"
+type: OpenSpec
+status: Draft
+domain: Reward Campaign Management
+---
+
+# Transaction Category
+
+## Requirement Definition
+A reusable group of conditions/criteria that define a specific category of transactions (e.g., "Dining transactions in Foreign Currency"). Allows efficient and reusable setup of Campaign Rule Criteria without needing multiple duplicated rules.
+
+## Process Flow
+
+### Pre-Conditions
+- Users must have access rights.
+- Criteria attributes must exist in the system.
+
+## Business Rules
+
+### Data Schema
+
+| Field Name | Description | Logic Type | Constraints / Rules |
+|---|---|---|---|
+| Transaction Category Code | Unique identifier. | String (5) | Required. |
+| Transaction Category Name | Name of category. | String (50) | Required. |
+| Description | Description for reference. | String (100) | Optional. |
+| Configuration | Groupings of criteria (AND/OR). | Rule Builder | Required. Same structure as Rule Criteria. |
+
+### Execution
+- Built using the Query Builder UI.
+- Supports nested AND/OR logic.
+- Includes NOT operators for exclusions.
+- A Transaction Category acts as a single virtual criterion when configuring a Campaign Rule.
+
+## Exception Flow
+- N/A