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>

backend/app/services/dify_client.py

@@ -3,7 +3,6 @@ Tool_OCR - DIFY AI Client
 HTTP client for DIFY translation API with batch support
 """
 
-import asyncio
 import logging
 import re
 import time
@@ -12,20 +11,10 @@ from typing import Dict, List, Optional
 
 import httpx
 
+from app.core.config import settings
+
 logger = logging.getLogger(__name__)
 
-# DIFY API Configuration
-DIFY_BASE_URL = "https://dify.theaken.com/v1"
-DIFY_API_KEY = "app-YOPrF2ro5fshzMkCZviIuUJd"
-DIFY_TIMEOUT = 120.0  # seconds (increased for batch)
-DIFY_MAX_RETRIES = 3
-
-# Batch translation limits
-# Conservative limits to avoid gateway timeouts
-# DIFY server may have processing time limits
-MAX_BATCH_CHARS = 5000
-MAX_BATCH_ITEMS = 20
-
 # Language name mapping
 LANGUAGE_NAMES = {
     "en": "English",
@@ -77,22 +66,39 @@ class DifyClient:
     - Blocking mode API calls
     - Automatic retry with exponential backoff
     - Token and latency tracking
+
+    Configuration is loaded from settings (config.py / .env.local):
+    - DIFY_BASE_URL: API base URL
+    - DIFY_API_KEY: API key (required)
+    - DIFY_TIMEOUT: Request timeout in seconds
+    - DIFY_MAX_RETRIES: Max retry attempts
+    - DIFY_MAX_BATCH_CHARS: Max characters per batch
+    - DIFY_MAX_BATCH_ITEMS: Max items per batch
     """
 
     def __init__(
         self,
-        base_url: str = DIFY_BASE_URL,
-        api_key: str = DIFY_API_KEY,
-        timeout: float = DIFY_TIMEOUT,
-        max_retries: int = DIFY_MAX_RETRIES
+        base_url: Optional[str] = None,
+        api_key: Optional[str] = None,
+        timeout: Optional[float] = None,
+        max_retries: Optional[int] = None
     ):
-        self.base_url = base_url
-        self.api_key = api_key
-        self.timeout = timeout
-        self.max_retries = max_retries
+        # Use settings as defaults when not explicitly provided
+        self.base_url = base_url or settings.dify_base_url
+        self.api_key = api_key or settings.dify_api_key
+        self.timeout = timeout if timeout is not None else settings.dify_timeout
+        self.max_retries = max_retries if max_retries is not None else settings.dify_max_retries
+        self.max_batch_chars = settings.dify_max_batch_chars
+        self.max_batch_items = settings.dify_max_batch_items
         self._total_tokens = 0
         self._total_requests = 0
 
+        # Warn if API key is not configured
+        if not self.api_key:
+            logger.warning(
+                "DIFY_API_KEY not configured. Set DIFY_API_KEY in .env.local for translation to work."
+            )
+
     def _get_language_name(self, lang_code: str) -> str:
         """Convert language code to full name for prompt"""
         return LANGUAGE_NAMES.get(lang_code, lang_code)