Fix SAM2 missing early watermarks: always sample first 15 seconds

For long videos (e.g. 1100s), uniform sampling started at ~23s, completely
missing watermarks that appear at 1s, 2s, 4s. Fix: always include dense
samples at 0.5, 1, 1.5, 2, 3, 4, 5, 7, 10, 15 seconds before the uniform
distribution, then deduplicate. Watermarks in the first 15s are now caught.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/tools/smart_detect.py

@@ -27,8 +27,19 @@ def detect_static_regions(video_path: str, sample_frames: int = 30, threshold: f
 
     duration_s = total / fps
     num_samples = max(10, min(24, int(duration_s // 8)))
-    positions = [min(int((i + 0.5) * total / num_samples), total - 1)
-                 for i in range(num_samples)]
+    uniform_positions = [min(int((i + 0.5) * total / num_samples), total - 1)
+                         for i in range(num_samples)]
+
+    # 水印常出现在视频开头几秒，强制加密采样前15秒
+    early_seconds = [0.5, 1, 1.5, 2, 3, 4, 5, 7, 10, 15]
+    early_positions = [min(int(s * fps), total - 1) for s in early_seconds if s * fps < total]
+    # 合并去重，保持顺序
+    seen = set()
+    positions = []
+    for p in early_positions + uniform_positions:
+        if p not in seen:
+            seen.add(p)
+            positions.append(p)
 
     bgr_frames = []
     gray_frames = []
@@ -40,7 +51,7 @@ def detect_static_regions(video_path: str, sample_frames: int = 30, threshold: f
             gray_frames.append(cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY).astype(np.float32))
     cap.release()
 
-    print(f"[SAM2-detect] 采样 {len(bgr_frames)} 帧 (均匀分布于{duration_s:.1f}s视频)", flush=True)
+    print(f"[SAM2-detect] 采样 {len(bgr_frames)} 帧 (前15秒密采+均匀分布，共{duration_s:.1f}s)", flush=True)
 
     if len(bgr_frames) < 4:
         print(f"[SAM2-detect] 帧数不足4帧，返回空", flush=True)