perf(review): 安装 Pillow 并优化截图下采样策略

- 视觉 token 成本由像素尺寸决定，故仅在长边超过 IMAGE_MAX_EDGE(默认1280) 时才缩放
- 不需要缩放且格式未变时原样返回，不重新编码，避免 CAD 线稿的无谓压缩损失
- 缩放后默认输出无损 PNG（IMAGE_FORMAT=png），可设 jpeg 换取更小体积
- 缺少 Pillow 时自动降级为原图直传

dwg_review/llm_client.py

@@ -23,28 +23,46 @@ class LlmError(Exception):
     pass
 
 
-def downsample_image_b64(b64, mime="image/png", max_edge=None, quality=None):
-    """把 base64 截图下采样并转 JPEG，降低视觉 token 成本。
+def downsample_image_b64(b64, mime="image/png", max_edge=None, fmt=None, quality=None):
+    """缩放截图，降低视觉 token 成本。
 
-    缺少 Pillow 或参数为 0 时原样返回 (b64, mime)。
+    策略（视觉 token 成本主要由**像素尺寸**决定，而非文件大小）：
+      - 仅当长边超过 `max_edge` 时才缩放；
+      - 既不需要缩放、目标格式又与原格式一致时，**原样返回**（不重新编码，
+        避免给 CAD 线稿引入无谓的压缩损失）；
+      - 需要缩放时按 `fmt` 编码：`png`（默认，无损，适合线稿）/ `jpeg`（体积更小）。
+
+    缺少 Pillow 或 max_edge <= 0 时原样返回 (b64, mime)。
     """
     max_edge = config.IMAGE_MAX_EDGE if max_edge is None else max_edge
+    fmt = (config.IMAGE_FORMAT if fmt is None else fmt).strip().lower()
     quality = config.IMAGE_JPEG_QUALITY if quality is None else quality
     if not HAS_PIL or max_edge <= 0:
         return b64, mime
+
+    target_mime = "image/jpeg" if fmt == "jpeg" else "image/png"
     try:
         img = Image.open(io.BytesIO(base64.b64decode(b64)))
-        if img.mode not in ("RGB", "L"):
-            img = img.convert("RGB")
         w, h = img.size
         scale = max_edge / float(max(w, h))
-        if scale < 1.0:
+        need_resize = scale < 1.0
+
+        if not need_resize and mime == target_mime:
+            return b64, mime
+
+        if need_resize:
             img = img.resize(
                 (max(1, int(w * scale)), max(1, int(h * scale))), Image.LANCZOS
             )
+
         buf = io.BytesIO()
-        img.save(buf, format="JPEG", quality=quality)
-        return base64.b64encode(buf.getvalue()).decode("ascii"), "image/jpeg"
+        if target_mime == "image/jpeg":
+            if img.mode not in ("RGB", "L"):
+                img = img.convert("RGB")
+            img.save(buf, format="JPEG", quality=quality)
+        else:
+            img.save(buf, format="PNG", optimize=True)
+        return base64.b64encode(buf.getvalue()).decode("ascii"), target_mime
     except Exception:
         return b64, mime