refactor: centralize DIFY settings in config.py and cleanup env files

- Update config.py to read both .env and .env.local (with .env.local priority)
- Move DIFY API settings from hardcoded values to environment configuration
- Remove unused PADDLEOCR_MODEL_DIR setting (models stored in ~/.paddleocr/)
- Remove deprecated argostranslate translation settings
- Add DIFY settings: base_url, api_key, timeout, max_retries, batch limits
- Update dify_client.py to use settings from config.py
- Update translation_service.py to use settings instead of constants
- Fix frontend env files to use correct variable name VITE_API_BASE_URL
- Update setup_dev_env.sh with correct PaddlePaddle version (3.2.0)

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

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

.env.example

@@ -31,7 +31,7 @@ TASK_RETENTION_DAYS=30
 MAX_TASKS_PER_USER=1000
 
 # ===== OCR Configuration =====
-PADDLEOCR_MODEL_DIR=./models/paddleocr
+# Note: PaddleOCR/PaddleX models are stored in ~/.paddleocr/ and ~/.paddlex/ by default
 OCR_LANGUAGES=ch,en,japan,korean
 OCR_CONFIDENCE_THRESHOLD=0.5
 MAX_OCR_WORKERS=4
@@ -69,10 +69,17 @@ PDF_MARGIN_RIGHT=20
 # ===== Translation Configuration (DIFY API) =====
 # Enable translation feature
 ENABLE_TRANSLATION=true
-# DIFY API endpoint
-DIFY_API_URL=https://your-dify-instance.example.com
+# DIFY API base URL
+DIFY_BASE_URL=https://your-dify-instance.example.com/v1
 # DIFY API key (get from DIFY dashboard)
 DIFY_API_KEY=your-dify-api-key
+# API request timeout in seconds
+DIFY_TIMEOUT=120.0
+# Maximum retry attempts
+DIFY_MAX_RETRIES=3
+# Batch translation limits
+DIFY_MAX_BATCH_CHARS=5000
+DIFY_MAX_BATCH_ITEMS=20
 
 # ===== Background Tasks Configuration =====
 TASK_QUEUE_TYPE=memory