### Technical Task: Fix Facebook Graph API Image Upload (403 Forbidden)

**Objective:**
Modify `FacebookPostingService.java` to resolve the `403 Forbidden` error caused by using a User Access Token for image uploads. Implement a mechanism to automatically exchange the User Token for a Page Access Token before attempting the upload.

**Context:**
The application is currently failing with this error:
`(#200) This endpoint is deprecated since the required permission publish_actions is deprecated`

This happens because we are trying to post to `/{userId}/photos` or using a User Token on a Page endpoint. Facebook requires a **Page Access Token** to post content. The user has valid permissions (`pages_read_engagement`, `pages_manage_posts`), but the code is not fetching the correct token type.

**Required Changes:**

1.  **Implement Token Exchange Method:**
    Create a helper method `getPageCredentials(String userAccessToken)` that:

    - Calls `GET /me/accounts?access_token={userToken}`.
    - Parses the JSON response to find the first available Page.
    - Extracts the `id` (Page ID) and `access_token` (Page Token).
    - Returns these credentials.

2.  **Update `postToFacebookWithImage`:**

    - Remove the logic that calls `getPageId` (which only returns the User ID or Page ID without a token).
    - Instead, call the new `getPageCredentials`.
    - Use the returned **Page Token** and **Page ID** for the subsequent `/photos` upload request.

3.  **Refactor `publishPostWithImage`:**

    - Ensure the `access_token` query parameter uses the **Page Token**, not the User Token passed into the service originally.
    - Keep the `ByteArrayResource` filename override (critical for multipart uploads).

**Code Reference (Implementation Details):**

Please replace the existing logic in `FacebookPostingService.java` with the following flow:

```java
// 1. New Helper Class
private static class PageCredentials {
    String pageId;
    String pageToken;
    public PageCredentials(String pageId, String pageToken) {
        this.pageId = pageId;
        this.pageToken = pageToken;
    }
}

// 2. New Method to fetch Page Token
private PageCredentials getPageCredentials(String userAccessToken) {
    // Call https://graph.facebook.com/v18.0/me/accounts
    // Return the first page found in the "data" array
}

// 3. Updated Main Method
public String postToFacebookWithImage(String userAccessToken, ...) {
    // Step A: Exchange tokens
    PageCredentials creds = getPageCredentials(userAccessToken);

    // Step B: Upload using creds.pageToken and creds.pageId
    return publishPostWithImage(creds.pageToken, creds.pageId, ...);
}
```

**Acceptance Criteria:**

- The service must successfully fetch a Page Access Token using the provided User Access Token.
- The image upload request must target `/{pageId}/photos`.
- The upload request must use the **Page Access Token**.
- The `403 Forbidden` error regarding `publish_actions` must no longer appear.