fix: resolve 7 frontend-backend API inconsistencies and add comprehensive documentation

- Fixed LoginResponse: added expires_in field
- Renamed format to file_format across FileInfo interface
- Updated ProcessRequest: replaced confidence_threshold with detect_layout
- Modified ProcessResponse: added message and total_files, removed task_id
- Removed non-existent getTaskStatus API call
- Fixed getOCRResult parameter from taskId to fileId
- Commented out translation config APIs pending backend implementation

Documentation:
- Added API_REFERENCE.md: Complete API endpoint inventory
- Added API_FIX_SUMMARY.md: Detailed before/after comparison of all fixes
- Added FRONTEND_API.md: Frontend integration documentation
- Added FRONTEND_QUICK_START.md: Quick start guide
- Added FRONTEND_UPGRADE_SUMMARY.md: Upgrade summary

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -6,7 +6,6 @@ import type {
   UploadResponse,
   ProcessRequest,
   ProcessResponse,
-  TaskStatus,
   BatchStatus,
   OCRResult,
   ExportRequest,
@@ -151,18 +150,11 @@ class ApiClient {
   }
 
   /**
-   * Get task status
+   * Get OCR result by file ID
+   * Note: Backend uses file-level tracking, not task-level
    */
-  async getTaskStatus(taskId: string): Promise<TaskStatus> {
-    const response = await this.client.get<TaskStatus>(`/ocr/status/${taskId}`)
-    return response.data
-  }
-
-  /**
-   * Get OCR result
-   */
-  async getOCRResult(taskId: string): Promise<OCRResult> {
-    const response = await this.client.get<OCRResult>(`/ocr/result/${taskId}`)
+  async getOCRResult(fileId: number): Promise<OCRResult> {
+    const response = await this.client.get<OCRResult>(`/ocr/result/${fileId}`)
     return response.data
   }
 
@@ -251,28 +243,26 @@ class ApiClient {
   }
 
   /**
-   * Get translation configs (STUB - Not yet implemented)
-   * This is a placeholder for future translation functionality
-   * @throws Will throw error with status 501 (Not Implemented)
+   * Get translation configs (NOT IMPLEMENTED)
+   * This endpoint does not exist on backend - configs will be part of Phase 5
+   * @deprecated Backend endpoint does not exist - will return 404
    */
-  async getTranslationConfigs(): Promise<TranslationConfig[]> {
-    // This endpoint is expected to return 501 Not Implemented until Phase 5
-    const response = await this.client.get<TranslationConfig[]>('/translate/configs')
-    return response.data
-  }
+  // async getTranslationConfigs(): Promise<TranslationConfig[]> {
+  //   const response = await this.client.get<TranslationConfig[]>('/translate/configs')
+  //   return response.data
+  // }
 
   /**
-   * Create translation config (STUB - Not yet implemented)
-   * This is a placeholder for future translation functionality
-   * @throws Will throw error with status 501 (Not Implemented)
+   * Create translation config (NOT IMPLEMENTED)
+   * This endpoint does not exist on backend - configs will be part of Phase 5
+   * @deprecated Backend endpoint does not exist - will return 404
    */
-  async createTranslationConfig(
-    config: Omit<TranslationConfig, 'id' | 'created_at'>
-  ): Promise<TranslationConfig> {
-    // This endpoint is expected to return 501 Not Implemented until Phase 5
-    const response = await this.client.post<TranslationConfig>('/translate/configs', config)
-    return response.data
-  }
+  // async createTranslationConfig(
+  //   config: Omit<TranslationConfig, 'id' | 'created_at'>
+  // ): Promise<TranslationConfig> {
+  //   const response = await this.client.post<TranslationConfig>('/translate/configs', config)
+  //   return response.data
+  // }
 }
 
 // Export singleton instance