fix: docs: backend setup - API Key index

[neooriginal]

This pull request adds an important setup step to the backend documentation, clarifying requirements for Firestore composite indexes. This will help prevent API key query failures and improve onboarding for new developers.

Documentation update:

• Added instructions to create composite indexes for dev_api_keys and mcp_api_keys collections in Firestore, specifying the required fields and their sort order. This is necessary to avoid internal server errors when loading API keys in the Developer Settings page.

[gemini-code-assist]

Code Review

This pull request adds an important documentation step for setting up the backend, specifically regarding the creation of Firestore composite indexes for API key queries. The change is clear and correctly identifies a necessary step to prevent errors. My review includes a suggestion to automate this setup process to make it more robust and less error-prone for developers, which I've classified as a high-severity improvement for project maintainability.

[gemini-code-assist]

Thank you for documenting this crucial setup step. To make the process more robust and less error-prone for new developers, I recommend automating the creation of these Firestore indexes. Instead of manual steps, you can define the indexes in a firestore.indexes.json file and have developers deploy them with a single command.

This involves:

1. Creating a firestore.indexes.json file in the backend/ directory.
2. Updating firebase.json to point to this file.
3. Replacing the manual instructions in the README with a command to deploy the indexes.

Here is the content for firestore.indexes.json:

{
  "indexes": [
    {
      "collectionGroup": "dev_api_keys",
      "queryScope": "COLLECTION",
      "fields": [
        { "fieldPath": "user_id", "order": "ASCENDING" },
        { "fieldPath": "created_at", "order": "DESCENDING" }
      ]
    },
    {
      "collectionGroup": "mcp_api_keys",
      "queryScope": "COLLECTION",
      "fields": [
        { "fieldPath": "user_id", "order": "ASCENDING" },
        { "fieldPath": "created_at", "order": "DESCENDING" }
      ]
    }
  ]
}

And in firebase.json:

{
  "firestore": {
    "indexes": "firestore.indexes.json"
  }
}

The suggested change below updates the README accordingly.

Suggested change

	- **Firestore Composite Indexes:** You must create composite indexes for API key queries. Go to [Firebase Console](https://console.firebase.google.com/) → Firestore Database → Indexes, and create the following composite indexes:
	| Collection | Fields |
	|------------|--------|
	| `dev_api_keys` | `user_id` (Ascending) + `created_at` (Descending) |
	| `mcp_api_keys` | `user_id` (Ascending) + `created_at` (Descending) |
	Without these indexes, the Developer Settings page will show "Failed to load API keys: Internal Server Error".
	- **Firestore Composite Indexes:** This project uses Firestore composite indexes for querying API keys. To deploy them, run the following command from the `backend` directory:
	```bash
	firebase deploy --only firestore:indexes
	```
	This command uses the definitions in `firestore.indexes.json`. Without these indexes, the Developer Settings page will show "Failed to load API keys: Internal Server Error".