docs: add guide for standard module structure patterns

docs/MODULE_STRUCTURE.md

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+# Aurora Module Structure Guide
+
+This guide documents the standard module organization patterns used in the Aurora codebase. Following these patterns ensures consistency, maintainability, and clear separation of concerns.
+
+## Module Anatomy
+
+A typical module in `@modules/` is organized into several files, each with a specific responsibility. 
+
+Example: `trade` module
+- `trade.service.ts`: Business logic and data access.
+- `trade.view.ts`: Discord UI components (embeds, modals, select menus).
+- `trade.interaction.ts`: Handler for interaction events (buttons, modals, etc.).
+- `trade.types.ts`: TypeScript interfaces and types.
+- `trade.service.test.ts`: Unit tests for the service logic.
+
+## File Responsibilities
+
+### 1. Service (`*.service.ts`)
+The core of the module. It contains the business logic, database interactions (using Drizzle), and state management.
+- **Rules**:
+  - Export a singleton instance: `export const tradeService = new TradeService();`
+  - Should not contain Discord-specific rendering logic (return data, not embeds).
+  - Throw `UserError` for validation issues that should be shown to the user.
+
+### 2. View (`*.view.ts`)
+Handles the creation of Discord-specific UI elements like `EmbedBuilder`, `ActionRowBuilder`, and `ModalBuilder`.
+- **Rules**:
+  - Focus on formatting and presentation.
+  - Takes raw data (from services) and returns Discord components.
+
+### 3. Interaction Handler (`*.interaction.ts`)
+The entry point for Discord component interactions (buttons, select menus, modals).
+- **Rules**:
+  - Export a single handler function: `export async function handleTradeInteraction(interaction: Interaction) { ... }`
+  - Routes internal `customId` patterns to specific logic.
+  - Relies on `ComponentInteractionHandler` for centralized error handling.
+  - **No local try-catch** for standard validation errors; let them bubble up as `UserError`.
+
+### 4. Types (`*.types.ts`)
+Central location for module-specific TypeScript types and constants.
+- **Rules**:
+  - Define interfaces for complex data structures.
+  - Use enums or literal types for states and custom IDs.
+
+## Interaction Routing
+
+All interaction handlers must be registered in `src/lib/interaction.routes.ts`. 
+
+```typescript
+{
+    predicate: (i) => i.customId.startsWith("module_"),
+    handler: () => import("@/modules/module/module.interaction"),
+    method: 'handleModuleInteraction'
+}
+```
+
+## Error Handling Standards
+
+Aurora uses a centralized error handling pattern in `ComponentInteractionHandler`.
+
+1. **UserError**: Use this for validation errors or issues the user can fix (e.g., "Insufficient funds").
+   - `throw new UserError("You need more coins!");`
+2. **SystemError / Generic Error**: Use this for unexpected system failures.
+   - These are logged to the console/logger and show a generic "Unexpected error" message to the user.
+
+## Naming Conventions
+
+- **Directory Name**: Lowercase, singular (e.g., `trade`, `inventory`).
+- **File Names**: `moduleName.type.ts` (e.g., `trade.service.ts`).
+- **Class Names**: PascalCase (e.g., `TradeService`).
+- **Service Instances**: camelCase (e.g., `tradeService`).
+- **Interaction Method**: `handle[ModuleName]Interaction`.