fix(T-002): remove sensitive info from logs

- alibaba.py: remove translated text content from log, only log char count
- nvidia.py: remove messages content reference from timing log
- push.py: replace full content with length, sanitize WeChat error response

backend/app/ai/providers/alibaba.py

@@ -67,7 +67,7 @@ class AlibabaMTProvider:
             detected = data.get("DetectedLanguage", src)
 
             if translated:
-                logger.info(f"Alibaba MT [{version}] ok: {text[:20]}... -> {translated[:20]}...")
+                logger.info(f"Alibaba MT [{version}] ok: {len(text)} chars translated")
                 return {
                     "translated_text": translated,
                     "provider": f"{self.name}-{version}",

backend/app/ai/providers/nvidia.py

@@ -45,7 +45,7 @@ class NvidiaProvider(OpenAIProvider):
 
         logger.info(
             f"NVIDIA timing: build_msgs={t1-t0:.1f}s api_call={t2-t1:.1f}s process={t3-t2:.1f}s "
-            f"chars_in={sum(len(m.get('content','')) for m in messages)} chars_out={len(content)}"
+            f"chars_out={len(content)}"
         )
 
         return {"reply": content, "provider": self.name, "model": self.model}