refactor: consolidate stream smoothing into StreamProcessor with intelligent timing

- Move StreamSmoother functionality directly into StreamProcessor for better integration
- Create ContentExtractor strategy pattern for format-agnostic content extraction
- Implement intelligent dynamic delay calculation based on text length
- Support three text length tiers: short (char-by-char), medium (chunked), long (chunked)
- Remove manual chunk_size and delay_ms configuration - now auto-calculated
- Simplify admin UI to single toggle switch with auto timing adjustment
- Extract format detection logic to reusable content_extractors module
- Improve code maintainability with cleaner architecture

src/services/system/config.py

@@ -81,15 +81,7 @@ class SystemConfigService:
         # 流式平滑输出配置
         "stream_smoothing_enabled": {
             "value": False,
-            "description": "是否启用流式平滑输出，将大 chunk 拆分成小块模拟打字效果",
-        },
-        "stream_smoothing_chunk_size": {
-            "value": 5,
-            "description": "流式平滑输出每个小块的字符数",
-        },
-        "stream_smoothing_delay_ms": {
-            "value": 15,
-            "description": "流式平滑输出每个小块之间的延迟毫秒数",
+            "description": "是否启用流式平滑输出，自动根据文本长度调整输出速度",
         },
     }