chore: 清理 generation.py 中未使用的渲染函数和常量

删除已废弃的 FFmpeg 渲染辅助函数（已被 RenderAdapter 统一路径替代）：
- _create_fallback_clip: 无素材时创建 fallback 视频
- _mux_audio_track: 后处理混音（视频+音频合并）
- _prepare_bgm_track: BGM 下载与准备

同时清理相关的未使用常量：
- OUTPUT_DURATION_SECONDS
- GENERATED_FILES_DIR
- FFMPEG_BIN, run_ffmpeg (from video_processing.ffmpeg_utils)
- import os

apps/worker/worker_app/tasks/generation.py

@@ -15,7 +15,6 @@ from __future__ import annotations
 
 import json
 import logging
-import os
 import tempfile
 import time
 from pathlib import Path
@@ -37,8 +36,6 @@ from packages.domain.bgm_utils import merge_bgm_config
 OUTPUT_WIDTH = 1280
 OUTPUT_HEIGHT = 720
 OUTPUT_FPS = 25.0
-OUTPUT_DURATION_SECONDS = 5.0
-GENERATED_FILES_DIR = Path(os.getenv("GENERATED_FILES_DIR", "/app/generated"))
 
 logger = logging.getLogger(__name__)
 
@@ -155,7 +152,6 @@ def _flush_logs(task_id: str, gen_task) -> None:
 # ── 共享工具模块导入 ──────────────────────────────────────────────────────────
 
 from video_processing.dedup_helpers import create_video_record_and_dedup
-from video_processing.ffmpeg_utils import FFMPEG_BIN, run_ffmpeg
 from video_processing.oss_helpers import (
     download_asset,
     get_signed_download_url,
@@ -330,61 +326,6 @@ def _build_plan_and_clips_from_task(
     return plan, clips, asset_path_map
 
 
-def _create_fallback_clip(output_path: Path, title: str) -> None:
-    """创建 fallback 视频（无素材时）"""
-    safe_title = title.replace(":", "\\:").replace("'", "\\'")[:80]
-    run_ffmpeg(
-        [
-            FFMPEG_BIN,
-            "-y",
-            "-f",
-            "lavfi",
-            "-i",
-            f"color=c=#111827:s={OUTPUT_WIDTH}x{OUTPUT_HEIGHT}:d={OUTPUT_DURATION_SECONDS}:r={int(OUTPUT_FPS)}",
-            "-vf",
-            f"drawtext=text='{safe_title}':fontcolor=white:fontsize=48:x=(w-text_w)/2:y=(h-text_h)/2",
-            "-c:v",
-            "libx264",
-            "-pix_fmt",
-            "yuv420p",
-            "-movflags",
-            "+faststart",
-            str(output_path),
-        ]
-    )
-
-
-def _mux_audio_track(video_path: Path, audio_path: str, output_path: Path) -> None:
-    """将音频轨混入已渲染的视频（后处理步骤）。
-
-    使用 FFmpeg 将视频和音频合并，视频时长为准，音频不足则循环，
-    音频过长则截断。
-    """
-    command = [
-        FFMPEG_BIN,
-        "-y",
-        "-i",
-        str(video_path),
-        "-i",
-        audio_path,
-        "-c:v",
-        "copy",
-        "-c:a",
-        "aac",
-        "-b:a",
-        "192k",
-        "-shortest",
-        "-map",
-        "0:v:0",
-        "-map",
-        "1:a:0",
-        "-movflags",
-        "+faststart",
-        str(output_path),
-    ]
-    run_ffmpeg(command)
-
-
 def _download_voice_asset(voice_library_id: str, local_path: Path) -> bool:
     """下载配音文件。
 
@@ -428,105 +369,6 @@ def _download_voice_asset(voice_library_id: str, local_path: Path) -> bool:
     return download_asset(storage_key, local_path)
 
 
-def _prepare_bgm_track(
-    *,
-    bgm_config: dict,
-    temp_path: Path,
-    task_id: str = "",
-) -> str | None:
-    """准备 BGM 音频文件（下载到本地）.
-
-    支持 3 种来源（按优先级）：
-    1. audio_url — 外部直链 URL（最高优先级）
-    2. asset_id — 素材库中的音频素材
-    3. preset_id — 预设 BGM 库
-
-    Returns:
-        BGM 本地文件路径，准备失败返回 None
-    """
-    from urllib.parse import urlparse
-
-    audio_url = bgm_config.get("audio_url", "") or ""
-    asset_id = bgm_config.get("asset_id", "") or ""
-    preset_id = bgm_config.get("preset_id", "") or ""
-
-    bgm_file = temp_path / f"bgm_{task_id or 'track'}.mp3"
-
-    # 优先级1：外部直链 URL
-    if audio_url:
-        try:
-            parsed = urlparse(audio_url)
-            if parsed.scheme in ("http", "https"):
-                from video_processing.url_security import (
-                    ALLOWED_AUDIO_MIME_TYPES,
-                    safe_download_file,
-                )
-
-                logger.info("[task_id=%s] [BGM] 从URL下载: %s", task_id, audio_url[:80])
-                safe_download_file(
-                    audio_url,
-                    str(bgm_file),
-                    purpose="bgm_download",
-                    allowed_mime_types=ALLOWED_AUDIO_MIME_TYPES,
-                    timeout=60.0,
-                )
-                if bgm_file.exists() and bgm_file.stat().st_size > 0:
-                    return str(bgm_file)
-        except Exception as e:
-            logger.warning("[task_id=%s] [BGM] URL下载失败: %s", task_id, e)
-
-    # 优先级2：素材库素材
-    if asset_id:
-        try:
-            from app.core.db import SessionLocal
-
-            from packages.adapters.sqlalchemy_impl.models import AssetModel
-
-            session = SessionLocal()
-            try:
-                model = session.query(AssetModel).filter(AssetModel.id == asset_id).first()
-                if model and (model.storage_key or model.file_url):
-                    # 兼容存量数据：storage_key 为空时 fallback 到 file_url
-                    storage_key = model.storage_key or model.file_url
-                    logger.info("[task_id=%s] [BGM] 从素材库下载: asset_id=%s", task_id, asset_id)
-                    ok = download_asset(storage_key, bgm_file)
-                    if ok and bgm_file.exists() and bgm_file.stat().st_size > 0:
-                        return str(bgm_file)
-            finally:
-                session.close()
-        except Exception as e:
-            logger.warning("[task_id=%s] [BGM] 素材库下载失败: %s", task_id, e)
-
-    # 优先级3：预设 BGM 库
-    if preset_id:
-        try:
-            from packages.domain.preset_bgm import get_preset_bgm
-
-            preset = get_preset_bgm(preset_id)
-            if preset and preset.audio_url:
-                from video_processing.url_security import (
-                    ALLOWED_AUDIO_MIME_TYPES,
-                    safe_download_file,
-                )
-
-                logger.info("[task_id=%s] [BGM] 从预设库下载: preset_id=%s", task_id, preset_id)
-                safe_download_file(
-                    preset.audio_url,
-                    str(bgm_file),
-                    purpose="bgm_preset_download",
-                    allowed_mime_types=ALLOWED_AUDIO_MIME_TYPES,
-                    timeout=60.0,
-                )
-                if bgm_file.exists() and bgm_file.stat().st_size > 0:
-                    return str(bgm_file)
-        except Exception as e:
-            logger.warning("[task_id=%s] [BGM] 预设库下载失败: %s", task_id, e)
-
-    # 所有来源都失败
-    logger.warning("[task_id=%s] [BGM] 所有来源都无法获取BGM，跳过", task_id)
-    return None
-
-
 def _verify_url_accessible(
     url: str,
     timeout: float = 10.0,