feat: 完善分割工作区导入导出与管理流程

- 新增基于 JWT 当前用户的登录恢复、角色权限、用户管理、审计日志和演示出厂重置后台接口与前端管理页。

- 重串 GT_label 导出和 GT Mask 导入逻辑：导出保留类别真实 maskid，导入仅接受灰度或 RGB 等通道 maskid 图，支持未知 maskid 策略、尺寸最近邻拉伸和导入预览。

- 统一分割结果导出体验：默认当前帧，按项目抽帧顺序和 XhXXmXXsXXXms 时间戳命名 ZIP 与图片，补齐 GT/Pro/Mix/分开 Mask 输出和映射 JSON。

- 调整工作区左侧工具栏：移除创建点/线段入口，新增画笔、橡皮擦及尺寸控制，并按绘制、布尔、导入/AI 工具分组分隔。

- 扩展 Canvas 编辑能力：画笔按语义分类绘制并可自动并入连通选中 mask，橡皮擦对选中区域扣除，优化布尔操作、选区、撤销重做和保存状态联动。

- 优化自动传播时间轴显示：同一蓝色系按传播新旧递进变暗，老传播记录达到阈值后统一旧记录色，并维护范围选择与清空后的历史显示。

- 将 AI 智能分割入口替换为更明确的 AI 元素图标，并同步侧栏、工作区和 AI 页面入口表现。

- 完善模板分类、maskid 工具函数、分类树联动、遮罩透明度、边缘平滑和传播链同步相关前端状态。

- 扩展后端项目、媒体、任务、Dashboard、模板和传播 runner 的用户隔离、任务控制、进度事件与兼容处理。

- 补充前后端测试，覆盖用户管理、GT_label 往返导入导出、GT Mask 校验和预览、画笔/橡皮擦、时间轴传播历史、导出范围、WebSocket 与 API 封装。

- 更新 AGENTS、README 和 doc 文档，记录当前接口契约、实现状态、测试计划、安装说明和 maskid/GT_label 规则。

backend/services/propagation_task_runner.py

@@ -102,8 +102,14 @@ def _normalize_smoothing_options(value: Any) -> dict[str, Any] | None:
     return {"strength": round(strength, 2), "method": method}
 
 
-def _chaikin_smooth_polygon(polygon: list[list[float]], iterations: int) -> list[list[float]]:
+def _smoothing_ratio(strength: float, curve: float = 1.65) -> float:
+    normalized = max(0.0, min(float(strength or 0.0), 100.0)) / 100.0
+    return normalized ** curve
+
+
+def _chaikin_smooth_polygon(polygon: list[list[float]], iterations: int, corner_cut: float = 0.25) -> list[list[float]]:
     points = _normalize_polygon(polygon)
+    q = max(0.02, min(float(corner_cut), 0.25))
     for _ in range(max(0, iterations)):
         if len(points) < 3:
             break
@@ -111,12 +117,12 @@ def _chaikin_smooth_polygon(polygon: list[list[float]], iterations: int) -> list
         for index, current in enumerate(points):
             following = points[(index + 1) % len(points)]
             next_points.append([
-                _clamp01(0.75 * current[0] + 0.25 * following[0]),
-                _clamp01(0.75 * current[1] + 0.25 * following[1]),
+                _clamp01((1.0 - q) * current[0] + q * following[0]),
+                _clamp01((1.0 - q) * current[1] + q * following[1]),
             ])
             next_points.append([
-                _clamp01(0.25 * current[0] + 0.75 * following[0]),
-                _clamp01(0.25 * current[1] + 0.75 * following[1]),
+                _clamp01(q * current[0] + (1.0 - q) * following[0]),
+                _clamp01(q * current[1] + (1.0 - q) * following[1]),
             ])
         points = next_points
     return points
@@ -127,7 +133,7 @@ def _simplify_polygon(polygon: list[list[float]], strength: float) -> list[list[
         return polygon
     contour = np.array([[[point[0], point[1]]] for point in polygon], dtype=np.float32)
     arc_length = cv2.arcLength(contour, True)
-    epsilon = arc_length * (0.001 + (strength / 100.0) * 0.006)
+    epsilon = arc_length * (0.00015 + _smoothing_ratio(strength) * 0.00735)
     approx = cv2.approxPolyDP(contour, epsilon, True).reshape(-1, 2)
     if len(approx) < 3:
         return polygon
@@ -140,9 +146,25 @@ def _smooth_polygon(polygon: list[list[float]], smoothing: dict[str, Any] | None
     strength = float(smoothing.get("strength") or 0.0)
     if strength <= 0:
         return _normalize_polygon(polygon)
-    iterations = max(1, min(3, int(strength // 35) + 1))
-    smoothed = _chaikin_smooth_polygon(polygon, iterations)
-    simplified = _simplify_polygon(smoothed, strength)
+    effective_strength = _smoothing_ratio(strength, curve=1.45) * 100.0
+    if effective_strength >= 85:
+        iterations = 4
+    elif effective_strength >= 55:
+        iterations = 3
+    elif effective_strength >= 25:
+        iterations = 2
+    else:
+        iterations = 1
+    corner_cut = 0.03 + _smoothing_ratio(strength, curve=1.35) * 0.22
+    normalized = _normalize_polygon(polygon)
+    pre_simplified = _simplify_polygon(normalized, effective_strength * 0.25)
+    smoothed = _chaikin_smooth_polygon(pre_simplified, iterations, corner_cut)
+    simplified = _simplify_polygon(smoothed, effective_strength)
+    if len(simplified) > len(normalized):
+        for fallback_strength in (25.0, 35.0, 50.0, 70.0, 90.0, 100.0):
+            simplified = _simplify_polygon(simplified, max(effective_strength, fallback_strength))
+            if len(simplified) <= len(normalized):
+                break
     return simplified if len(simplified) >= 3 else _normalize_polygon(polygon)