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/library/Add Library Folder.bru

@@ -5,7 +5,7 @@ meta {
 }
 
 post {
-  url: {{base_url}}/api/libraries/{{libraryId}}/folders
+  url: {{base_url}}/api/libraries/{{library_id}}/folders
   auth: inherit
   body: json
 }

bruno/library/Get Library Folders.bru

@@ -5,7 +5,7 @@ meta {
 }
 
 get {
-  url: {{base_url}}/api/libraries/{{libraryId}}/folders
+  url: {{base_url}}/api/libraries/{{library_id}}/folders
   auth: inherit
 }
 

bruno/library/Set Library Visibility.bru

@@ -16,8 +16,8 @@ headers {
 
 body:json {
   {
-    "library_id": "{{libraryId}}",
-    "is_visible": {{isVisible}}
+    "library_id": "{{library_id}}",
+    "is_visible": {{is_visible}}
   }
 }