feat(diagrams): add Mermaid diagrams for backend architecture, API map, and use-case flows

docs/prompts/create-mermaid-be-diagrams-flutter.md

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+## What the prompt does
+This prompt generates a Mermaid diagram that visualizes the backend architecture of a Flutter project. It uses the given overview and use case diagrams to create a detailed diagram that shows the relationships between different components and services.
+
+## Assumption
+- Flutter project is given
+- Overview mermaid diagram is given
+- Use case diagram is given
+
+## How to use the prompt
+For the given Flutter project, the backend uses **GraphQL** and **Firebase**. I want multiple detailed Mermaid diagrams to understand how everything is connected.
+
+Please do the following:
+
+1. **Read and analyze** the entire project, along with these two files:
+
+   * `overview.mermaid`
+   * `use-case-flowchart.mermaid`
+
+2. Based on all available information, generate **three separate Mermaid diagrams**:
+
+   ### A. Backend Architecture Diagram
+
+   * Show the high-level structure of the backend.
+   * Include GraphQL server components, Firebase services (Auth, Firestore, Storage, Functions, etc.), and how the Flutter app connects to them.
+   * Show data flow between Flutter → GraphQL → Firebase → back to the client.
+
+   ### B. API Map (GraphQL Operations + Firebase Interactions)
+
+   * List and group all GraphQL queries, mutations, and subscriptions.
+   * Show which ones interact with Firebase and how.
+   * If Firestore collections or documents are involved, show them as nodes.
+   * Clearly illustrate the relationship between API operations and backend resources.
+
+   ### C. Use-Case Flow Diagrams
+
+   * For each major use case in the project:
+
+     * Show how the request moves from the Flutter UI to the backend.
+     * Show the sequence of steps involving GraphQL operations and Firebase services.
+     * Show how responses return back to the UI.
+   * Organize all use cases into **one combined Mermaid diagram** or **multiple subgraph clusters**.
+
+3. Ensure all diagrams are:
+
+   * Clean, readable, and logically grouped
+   * Consistent with the structure of the existing project and the two Mermaid reference files
+   * Detailed enough for developers to understand backend behavior at a glance
+
+4. Output the three diagrams clearly labeled as:
+
+   * **Backend Architecture**
+   * **API Map**
+   * **Use-Case Flows**
+
+---
+
+If you'd like, I can also generate a **Cursor-optimized** version or a **Claude Projects** version.