refactor: remove_small_objects用Otsu替代中位数25%

对连通域面积分布做Otsu自动找分界，不再拍脑袋定百分比

web/app.py

@@ -72,11 +72,18 @@ def keep_largest_object(binary):
 def remove_small_objects(binary):
     L, n = ndimage.label(binary)
     if n == 0: return binary
-    areas = [int(np.sum(L==i)) for i in range(1,n+1)]
-    minsz = max(1, int(np.median(areas)*0.25))
+    areas = np.array([int(np.sum(L==i)) for i in range(1,n+1)])
+    if len(areas) < 2: return binary
+    best_T, best_cost, n_total = 0, float('inf'), len(areas)
+    for T in np.unique(areas):
+        s, l = areas[areas<=T], areas[areas>T]
+        w_s, w_l = len(s)/n_total, len(l)/n_total
+        if w_s==0 or w_l==0: continue
+        cost = w_s*np.var(s) + w_l*np.var(l)
+        if cost < best_cost: best_cost, best_T = cost, T
     r = binary.copy()
     for i in range(1, n+1):
-        if areas[i-1] < minsz: r[L==i] = 0
+        if int(np.sum(L==i)) < best_T: r[L==i] = 0
     return r