feat: 统一渲染引擎 + 打通一键生成全链路

核心变更：
1. 提取共享工具模块（ffmpeg_utils / oss_helpers / dedup_helpers）
2. 实现 UnifiedRenderService — 按 clip_type/config.role 分组为图层再合成
3. 重构 render_edit_plan() 使用 UnifiedRenderService（替换 concat demuxer）
4. 重构 generate_video() 使用 UnifiedRenderService（替换 EditingModeProcessor）
5. 集成 VideoDeduplicator 查重
6. 补 23 个单元测试 + 14 个四模式集成测试 + 6 个全链路测试

图层分组算法：
  main → main (z=0)
  main+config.role=b_roll → broll (z=0)
  overlay → overlay (z=1)
  background → background (z=-1)
  corner_voice → corner_voice (z=1)
  b_roll → broll (z=0)
  intro/outro → main (z=0)

合成流程：每个 clip 预处理 → 同层 xfade 串联 → overlay 合成 → 音频混入

apps/worker/video_processing/ffmpeg_utils.py

@@ -0,0 +1,289 @@
+"""FFmpeg 工具函数 — 从 editing_modes.py / video_compose_service.py 提取的共享原语.
+
+提供 FFmpeg / FFprobe 调用、视频信息探测、视频标准化、xfade 转场滤镜构建
+等底层能力，供 EditingModeProcessor、VideoComposeService、UnifiedRenderService
+共同复用。
+"""
+
+from __future__ import annotations
+
+import logging
+import shutil
+import subprocess  # nosec B404
+from pathlib import Path
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+# ── 常量 ──────────────────────────────────────────────────────────────────────
+
+FFMPEG_BIN: str = shutil.which("ffmpeg") or "ffmpeg"
+FFPROBE_BIN: str = shutil.which("ffprobe") or "ffprobe"
+
+DEFAULT_OUTPUT_WIDTH = 1280
+DEFAULT_OUTPUT_HEIGHT = 720
+DEFAULT_FPS = 25
+
+# xfade 转场映射：transition_effect 名称 → FFmpeg xfade transition 名称
+# 键同时支持 TransitionEffect 枚举值和字符串名称（向后兼容）
+XFADE_TRANSITION_MAP: dict[str, str] = {
+    "fade": "fade",
+    "slideleft": "slideleft",
+    "slide_left": "slideleft",
+    "slideright": "slideright",
+    "slide_right": "slideright",
+    "dissolve": "dissolve",
+    "wipe": "wipeleft",
+    "wipeleft": "wipeleft",
+}
+
+DEFAULT_TRANSITION_DURATION = 0.5
+
+
+# ── FFmpeg 执行 ───────────────────────────────────────────────────────────────
+
+
+def run_ffmpeg(
+    command: list[str],
+    *,
+    capture_output: bool = True,
+) -> tuple[str, str]:
+    """执行 FFmpeg 命令。
+
+    Args:
+        command: 完整的 ffmpeg 命令列表（含 "ffmpeg" 本身）
+        capture_output: 是否捕获 stdout/stderr
+
+    Returns:
+        (stdout, stderr) 元组
+
+    Raises:
+        subprocess.CalledProcessError: 命令执行失败时抛出
+    """
+    result = subprocess.run(  # nosec B603
+        command,
+        check=True,
+        stdout=subprocess.PIPE if capture_output else None,
+        stderr=subprocess.PIPE if capture_output else None,
+        text=True,
+    )
+    return (result.stdout or "", result.stderr or "")
+
+
+def probe_duration(local_path: str | Path) -> float:
+    """用 ffprobe 获取视频时长（秒）。
+
+    失败时返回默认值 5.0 秒。
+    """
+    try:
+        result = subprocess.run(  # nosec B603
+            [
+                FFPROBE_BIN,
+                "-v",
+                "error",
+                "-show_entries",
+                "format=duration",
+                "-of",
+                "default=noprint_wrappers=1:nokey=1",
+                str(local_path),
+            ],
+            check=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            text=True,
+        )
+        return round(float(result.stdout.strip()), 3)
+    except Exception:
+        return 5.0
+
+
+def probe_video_info(video_path: str) -> dict[str, Any]:
+    """获取视频信息（宽、高、时长、fps）。
+
+    Returns:
+        {"width": int, "height": int, "duration": float, "fps": float}
+        失败时返回默认值。
+    """
+    try:
+        result = subprocess.run(  # nosec B603
+            [
+                FFPROBE_BIN,
+                "-v",
+                "error",
+                "-select_streams",
+                "v:0",
+                "-show_entries",
+                "stream=width,height,r_frame_rate,duration",
+                "-show_entries",
+                "format=duration",
+                "-of",
+                "json",
+                video_path,
+            ],
+            check=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            text=True,
+        )
+
+        import json
+
+        info = json.loads(result.stdout)
+        stream = info.get("streams", [{}])[0]
+        fmt = info.get("format", {})
+
+        width = int(stream.get("width", DEFAULT_OUTPUT_WIDTH))
+        height = int(stream.get("height", DEFAULT_OUTPUT_HEIGHT))
+
+        # 解析帧率
+        fps_str = stream.get("r_frame_rate", "25/1")
+        if "/" in fps_str:
+            num, den = fps_str.split("/")
+            fps = float(num) / float(den) if float(den) > 0 else DEFAULT_FPS
+        else:
+            fps = float(fps_str) if fps_str else DEFAULT_FPS
+
+        # 时长
+        duration = float(fmt.get("duration", 0)) or float(stream.get("duration", 0))
+
+        return {
+            "width": width,
+            "height": height,
+            "duration": duration,
+            "fps": round(fps, 2),
+        }
+    except Exception as e:
+        logger.warning("获取视频信息失败: %s, error: %s", video_path, e)
+        return {
+            "width": DEFAULT_OUTPUT_WIDTH,
+            "height": DEFAULT_OUTPUT_HEIGHT,
+            "duration": 0.0,
+            "fps": DEFAULT_FPS,
+        }
+
+
+def normalize_video(
+    input_path: str,
+    output_path: str,
+    *,
+    width: int = DEFAULT_OUTPUT_WIDTH,
+    height: int = DEFAULT_OUTPUT_HEIGHT,
+    fps: int = DEFAULT_FPS,
+) -> dict[str, Any]:
+    """标准化视频（缩放 + 恒定帧率）。
+
+    使用 scale + pad 保持宽高比，黑边填充到目标分辨率。
+
+    Returns:
+        {"width": int, "height": int, "path": str}
+    """
+    command = [
+        FFMPEG_BIN,
+        "-y",
+        "-i",
+        input_path,
+        "-vf",
+        f"scale={width}:{height}:force_original_aspect_ratio=decrease,"
+        f"pad={width}:{height}:(ow-iw)/2:(oh-ih)/2:black,"
+        f"fps={fps}",
+        "-c:v",
+        "libx264",
+        "-crf",
+        "23",
+        "-preset",
+        "medium",
+        "-c:a",
+        "aac",
+        "-b:a",
+        "128k",
+        "-movflags",
+        "+faststart",
+        output_path,
+    ]
+    run_ffmpeg(command)
+    return {"width": width, "height": height, "path": output_path}
+
+
+# ── xfade / concat 滤镜构建 ──────────────────────────────────────────────────
+
+
+def chain_filters(filters: list[str], output_label: str, *, input_label: str = "0:v") -> str:
+    """将滤镜列表串联为 FFmpeg 滤镜字符串。
+
+    例：chain_filters(["scale=1280:720", "fps=25"], "v0")
+    → "[0:v]scale=1280:720,fps=25[v0]"
+    """
+    filter_body = ",".join(filters)
+    return f"[{input_label}]{filter_body}[{output_label}]"
+
+
+def resolve_xfade_transition(transition_name: str) -> str:
+    """将转场效果名称映射为 FFmpeg xfade transition 名称。
+
+    支持 TransitionEffect 枚举值和字符串名称，未知值回退到 "fade"。
+    """
+    # 兼容 TransitionEffect 枚举（有 .value 属性）
+    if hasattr(transition_name, "value"):
+        transition_name = transition_name.value
+    return XFADE_TRANSITION_MAP.get(transition_name, "fade")
+
+
+def build_xfade_filter_chain(
+    clip_durations: list[float],
+    clip_video_labels: list[str],
+    transitions: list[str],
+    *,
+    transition_duration: float = DEFAULT_TRANSITION_DURATION,
+    output_label: str = "outv",
+) -> tuple[str, float]:
+    """构建 xfade 转场滤镜链。
+
+    Args:
+        clip_durations: 每个片段的时长
+        clip_video_labels: 每个片段的视频流标签（如 "v0", "v1"）
+        transitions: 每个片段对应的转场效果（第一个片段的转场被忽略）
+        transition_duration: 转场时长（秒）
+        output_label: 最终输出标签
+
+    Returns:
+        (filter_string, estimated_total_duration)
+    """
+    n = len(clip_durations)
+    parts: list[str] = []
+    total_duration = sum(clip_durations)
+
+    if n == 0:
+        return "", 0.0
+
+    if n == 1:
+        parts.append(f"[{clip_video_labels[0]}]copy[{output_label}]")
+        return ";".join(parts), total_duration
+
+    # xfade 链
+    cumulative = 0.0
+    prev_label = clip_video_labels[0]
+
+    for i in range(1, n):
+        cumulative += clip_durations[i - 1]
+        offset = max(0.0, cumulative - transition_duration * i)
+
+        transition = transitions[i] if i < len(transitions) else "cut"
+        xfade_transition = resolve_xfade_transition(transition)
+
+        if i == n - 1:
+            out_label = output_label
+        else:
+            out_label = f"xf{i}"
+
+        parts.append(
+            f"[{prev_label}][{clip_video_labels[i]}]"
+            f"xfade=transition={xfade_transition}"
+            f":duration={transition_duration}"
+            f":offset={offset:.3f}"
+            f"[{out_label}]"
+        )
+        prev_label = out_label
+
+    # 总时长减去转场重叠部分
+    total_duration -= transition_duration * (n - 1)
+    return ";".join(parts), max(0.0, total_duration)