docs: Write quetza docs

services/docs/content/en/quetza/concepts/module-system.md

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+---
+title: The Module System
+weight: 2
+---
+
+
+**Goal:** Define the vocabulary and mechanics of Quetza's primary abstraction unit.
+
+## System Vocabulary
+
+### Module Declaration
+The `module.ts` file acts as the manifest. It is the entry point for discovery.
+```typescript
+export const name = 'my-module'; // ID
+export const description = '...'; // Doc
+```
+
+### Command Registration
+Occurs during the `ready` event.
+1.  Quetza scans all `commands/` folders.
+2.  Compiles `ApplicationCommandData`.
+3.  PUTs the payload to Discord's API.
+
+### Controller Injection
+State is injected, not imported.
+```typescript
+// The framework calls this:
+execute(client, interaction, controller)
+```
+*   **Why?** Allows the framework to manage the controller's lifecycle (singleton per module).
+
+## State Management Philosophy
+
+### Ephemeral by Default
+Quetza provides **no built-in database**.
+*   **Memory:** Controllers store state in JS `Map` or `Set` objects.
+*   **Restart:** All state is lost when the process exits.
+*   **Persistence:** Must be implemented manually (e.g., SQLite, Redis) inside the Controller.
+
+### Scope as Key
+State must be keyed by its boundary.
+*   `Map<GuildID, ...>` - Server specific (Most common).
+*   `Map<UserID, ...>` - User specific (DM bots).
+
+## Error Handling Model
+
+**Fail Fast & Friendly.**
+*   **Validation:** Check permissions/inputs immediately.
+*   **User-Facing:** "You don't have permission" (Not "Error: 403").
+*   **Isolation:** A crash in `/ping` catches itself and doesn't kill the bot.
+
+---
+**See also:**
+- [Discord Interaction Lifecycle](https://discord.com/developers/docs/interactions/receiving-and-responding)