fix: use atrim filter for precise+efficient audio trimming in simple path

Replace -ss output seeking (which decodes all data before trim point)
with atrim filter in -af chain. Filter order: atrim -> asetpts -> volume,
ensuring volume is only applied to the retained segment.

When no trimming needed, use direct extraction for maximum efficiency.

apps/worker/video_processing/render_audio.py

@@ -279,37 +279,68 @@ def concat_main_audio(
         has_speed = abs(speed - 1.0) >= 1e-6
 
         if not has_speed and not has_reverse:
-            # 无调速无倒放：简单命令行。
-            # 注意：-ss 放在 -i 之后（输出选项），逐帧解码到精确位置，保证采样点级精度。
-            # 有调速或倒放时走下方 else 分支（filter_complex），那里用 atrim 精确裁剪。
-            command = [
-                FFMPEG_BIN,
-                "-y",
-                "-i",
-                str(clip.local_path),
-                "-vn",
-                "-acodec",
-                "aac",
-                "-b:a",
-                "128k",
-                "-ar",
-                "48000",
-                "-ac",
-                "2",
-            ]
-            # 单 clip 简单模式：将所有音频滤镜合并为单个 -af 参数（避免多个 -af 冲突）
-            af_parts: list[str] = []
+            # 无调速无倒放：根据是否需要裁剪/音量选择最高效的路径。
             vol = _clip_volume(clip)
-            if abs(vol - 1.0) >= 1e-6:
-                af_parts.append(f"volume={vol:.4f}")
-            if af_parts:
-                command.extend(["-af", ",".join(af_parts)])
-            if trim_start > 0:
-                command.extend(["-ss", f"{trim_start:.3f}"])
-            if final_duration > 0:
-                command.extend(["-t", f"{final_duration:.3f}"])
-            command.append(str(output_path))
-            run_ffmpeg(command)
+            need_trim = trim_start > 0 or (effective_duration > 0 and final_duration < adjusted_duration)
+            need_volume = abs(vol - 1.0) >= 1e-6
+
+            if need_trim:
+                # 需要裁剪：用 atrim 滤镜在滤镜链中精确裁剪（采样点级精度，不浪费解码）。
+                # 滤镜顺序：atrim → asetpts → volume（先裁剪再调音量，避免处理被丢弃的数据）。
+                af_parts: list[str] = []
+                if trim_start > 0 and effective_duration > 0:
+                    af_parts.append(f"atrim=start={trim_start:.3f}:duration={final_duration:.3f}")
+                elif trim_start > 0:
+                    af_parts.append(f"atrim=start={trim_start:.3f}")
+                elif final_duration > 0:
+                    af_parts.append(f"atrim=duration={final_duration:.3f}")
+                af_parts.append("asetpts=PTS-STARTPTS")
+                if need_volume:
+                    af_parts.append(f"volume={vol:.4f}")
+                command = [
+                    FFMPEG_BIN,
+                    "-y",
+                    "-i",
+                    str(clip.local_path),
+                    "-vn",
+                    "-af",
+                    ",".join(af_parts),
+                    "-acodec",
+                    "aac",
+                    "-b:a",
+                    "128k",
+                    "-ar",
+                    "48000",
+                    "-ac",
+                    "2",
+                ]
+                if final_duration > 0 and trim_start <= 0:
+                    command.extend(["-t", f"{final_duration:.3f}"])
+                command.append(str(output_path))
+                run_ffmpeg(command)
+            else:
+                # 无需裁剪：直接提取，最高效。音量用单个 -af（如有）。
+                command = [
+                    FFMPEG_BIN,
+                    "-y",
+                    "-i",
+                    str(clip.local_path),
+                    "-vn",
+                    "-acodec",
+                    "aac",
+                    "-b:a",
+                    "128k",
+                    "-ar",
+                    "48000",
+                    "-ac",
+                    "2",
+                ]
+                if need_volume:
+                    command.extend(["-af", f"volume={vol:.4f}"])
+                if final_duration > 0:
+                    command.extend(["-t", f"{final_duration:.3f}"])
+                command.append(str(output_path))
+                run_ffmpeg(command)
         else:
             # 有调速或倒放：用 filter_complex
             speed_engine = SpeedEngine()