refactor(bruno): standardize all variables to snake_case naming convention

Standardize all Bruno environment variables to use snake_case convention
(aligned with Go naming practices) and remove duplicate camelCase variants.

Changes:
- Environment file cleanup:
  - Remove: baseUrl, ebookid, fakebookid, libraryId, mediaItemId, isVisible, refreshToken
  - Standardize: fakebookid → fake_book_id, isVisible → is_visible, refreshToken → refresh_token
  - All variables now use consistent snake_case format

- Update all Bruno requests to use standardized variables:
  - ebooks: {{ebookid}} → {{ebook_id}}
  - library: {{libraryId}} → {{library_id}}
  - media-items: {{mediaItemId}} → {{media_item_id}}
  - visibility: {{isVisible}} → {{is_visible}}
  - auth: {{refreshToken}} → {{refresh_token}}

Benefits:
- Single source of truth for each variable
- Consistent with Go naming conventions
- No ambiguity about which variable name to use
- Cleaner, more maintainable codebase

bruno/progress/Get Reading Progress.bru

@@ -5,7 +5,7 @@ meta {
 }
 
 get {
-  url: {{base_url}}/api/ebooks/{{ebookid}}/progress
+  url: {{base_url}}/api/ebooks/{{ebook_id}}/progress
   body: none
   auth: inherit
 }

bruno/progress/Update Reading Progress.bru

@@ -5,7 +5,7 @@ meta {
 }
 
 put {
-  url: {{base_url}}/api/ebooks/{{ebookid}}/progress
+  url: {{base_url}}/api/ebooks/{{ebook_id}}/progress
   body: json
   auth: inherit
 }