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

docs/specs/reward_campaign/reward_campaign.md

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+---
+title: "Reward Campaign"
+type: OpenSpec
+status: Draft
+domain: Reward Campaign Management
+---
+
+# Reward Campaign
+
+## Requirement Definition
+Acts as the parent container for Business Rules (Campaign Rules) controlling rewards and redemptions. Campaigns can be "Base" (core product) or "Tactical" (short-term objectives).
+
+## Process Flow
+
+### Pre-Conditions
+- Users must have access rights.
+
+## Business Rules
+
+### Data Schema
+
+| Field Name | Description | Logic Type | Constraints / Rules |
+|---|---|---|---|
+| Campaign ID | Identifier. | String (10) | Required. |
+| Campaign Name | Name of the campaign. | String (50) | Required. >= 10 chars. |
+| Campaign Owner | Owner reference. | String (50) | Optional. |
+| Campaign Description | Description. | String (500) | Optional. |
+| Campaign Type | Base or Tactical. | Enum (Lookup) | Required. |
+| Target Active Customer Count | Expected participant count. | Integer | Optional. Insight metric. |
+| Target Avg Transaction Value | Expected spend value. | Decimal | Optional. Insight metric. |
+| Campaign Period | Earliest Rule start to latest Rule end. | Date Range | Read-only based on Rules. |
+
+### Execution
+- Campaigns simply group Rules logically. The actual execution logic is defined in the child Campaign Rules.
+
+## Exception Flow
+- N/A