docs: add comprehensive device authentication implementation details

- Add IMPLEMENTATION_EXACT.md with exact code changes for all phases
- Update IMPLEMENTATION_PLAN.md with clarifications on two-field approach:
  - device_identifier: Serial number (Kobo) or UUID (KOReader)
  - auth_token: Auto-generated API key for authentication
- Resolve all user questions with ✅ marked decisions
- Add verification steps for documentation accuracy
- Document Kobo vs KOReader registration workflow differences
- Add SQL query for token regeneration (UpdateDeviceAuthToken)
- Include TypeScript device management code
- Add Bruno API test files for all new endpoints
- Update Kobo setup documentation for URL path token approach

bruno/environments/Bookhoard.bru

@@ -10,8 +10,11 @@ vars {
   rating: 5
   is_visible: true
   library_folder: /app/uploads
+  opds_base_url: 
 }
 vars:secret [
   token,
-  refresh_token
+  refresh_token,
+  kobo_device_token,
+  other_device_id
 ]