feat: add GPU acceleration support OpenSpec proposal

新增 GPU 加速支援的 OpenSpec 變更提案

主要內容：
- 在環境建置腳本中加入 GPU 偵測功能
- 自動安裝對應 CUDA 版本的 PaddlePaddle GPU 套件
- 在 OCR 處理程式中加入 GPU 可用性偵測
- 自動啟用 GPU 加速（可用時）或使用 CPU（不可用時）
- 支援強制 CPU 模式選項
- 加入 GPU 狀態報告到健康檢查 API

變更範圍：
- 新增 capability: environment-setup (環境設置)
- 修改 capability: ocr-processing (加入 GPU 支援)

實作任務包含：
1. 環境設置腳本增強 (GPU 偵測、CUDA 安裝)
2. 配置更新 (GPU 相關環境變數)
3. OCR 服務 GPU 整合 (自動偵測、記憶體管理)
4. 健康檢查與監控 (GPU 狀態報告)
5. 文檔更新
6. 測試與效能評估
7. 錯誤處理與邊界情況

預期效果：
- GPU 系統: 3-10x OCR 處理速度提升
- CPU 系統: 無影響，向後相容
- 自動硬體偵測與優化配置

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

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

openspec/changes/add-gpu-acceleration-support/specs/ocr-processing/spec.md

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+# OCR Processing Specification
+
+## ADDED Requirements
+
+### Requirement: GPU Acceleration
+The system SHALL automatically detect and utilize GPU hardware for OCR processing when available, with graceful fallback to CPU mode when GPU is unavailable or disabled.
+
+#### Scenario: GPU available and enabled
+- **WHEN** PaddleOCR service initializes on system with compatible GPU
+- **THEN** the system detects GPU availability using CUDA runtime
+- **AND** initializes PaddleOCR with `use_gpu=True` parameter
+- **AND** sets appropriate GPU memory fraction to prevent OOM errors
+- **AND** logs GPU device information (name, memory, CUDA version)
+- **AND** processes OCR tasks using GPU acceleration
+
+#### Scenario: CPU fallback when GPU unavailable
+- **WHEN** PaddleOCR service initializes on system without GPU
+- **THEN** the system detects absence of GPU
+- **AND** initializes PaddleOCR with `use_gpu=False` parameter
+- **AND** logs CPU mode status
+- **AND** processes OCR tasks using CPU without errors
+
+#### Scenario: Force CPU mode override
+- **WHEN** FORCE_CPU_MODE environment variable is set to true
+- **THEN** the system ignores GPU availability
+- **AND** initializes PaddleOCR in CPU mode
+- **AND** logs that CPU mode is forced by configuration
+- **AND** processes OCR tasks using CPU
+
+#### Scenario: GPU out-of-memory error handling
+- **WHEN** GPU runs out of memory during OCR processing
+- **THEN** the system catches CUDA OOM exception
+- **AND** logs error with GPU memory information
+- **AND** attempts to process the task using CPU mode
+- **AND** continues batch processing without failure
+- **AND** records GPU failure in task metadata
+
+#### Scenario: Multiple GPU devices available
+- **WHEN** system has multiple CUDA devices
+- **THEN** the system detects all available GPUs
+- **AND** uses primary GPU (device 0) by default
+- **AND** allows GPU device selection via configuration
+- **AND** logs selected GPU device information
+
+### Requirement: GPU Performance Optimization
+The system SHALL optimize GPU memory usage and batch processing for efficient OCR performance.
+
+#### Scenario: Automatic batch size adjustment
+- **WHEN** GPU mode is enabled
+- **THEN** the system queries available GPU memory
+- **AND** calculates optimal batch size based on memory capacity
+- **AND** adjusts concurrent processing threads accordingly
+- **AND** monitors memory usage during processing
+- **AND** prevents memory allocation beyond safe threshold
+
+#### Scenario: GPU memory management
+- **WHEN** GPU memory fraction is configured
+- **THEN** the system allocates specified fraction of total GPU memory
+- **AND** reserves memory for PaddleOCR model
+- **AND** prevents other processes from causing OOM
+- **AND** releases memory after batch completion
+
+### Requirement: GPU Status Reporting
+The system SHALL provide GPU status information through health check API and logging.
+
+#### Scenario: Health check with GPU available
+- **WHEN** client requests `/health` endpoint on GPU-enabled system
+- **THEN** the system returns health status including:
+  - `gpu_available`: true
+  - `gpu_device_name`: detected GPU name
+  - `cuda_version`: CUDA runtime version
+  - `gpu_memory_total`: total GPU memory in MB
+  - `gpu_memory_used`: currently used GPU memory in MB
+  - `gpu_utilization`: current GPU utilization percentage
+
+#### Scenario: Health check without GPU
+- **WHEN** client requests `/health` endpoint on CPU-only system
+- **THEN** the system returns health status including:
+  - `gpu_available`: false
+  - `processing_mode`: "CPU"
+  - `reason`: explanation for CPU mode (e.g., "No GPU detected", "CPU mode forced")
+
+#### Scenario: Startup GPU status logging
+- **WHEN** OCR service starts
+- **THEN** the system logs GPU detection results
+- **AND** logs selected processing mode (GPU/CPU)
+- **AND** logs GPU device details if available
+- **AND** logs any GPU-related warnings or errors
+- **AND** continues startup successfully regardless of GPU status