fix(api): use celery_app.send_task() for viral_video enqueue instead of direct worker import

The viral_video API route module had 4 places that did
`from worker_app.tasks.viral_video import <task>` and then <task>.delay().
In the API container the worker_app package is not installed, so calling
generate/retry/confirm-intent/analyze-style would raise ModuleNotFoundError:
No module named 'worker_app'.

Switched all four call sites to celery_app.send_task() string-dispatch
(matches the existing pattern in app/core/task_enqueue.py for
worker.generate_video). Verified task names match @shared_task(name=...)
declared in worker_app/tasks/viral_video.py:
  - worker.run_viral_video_pipeline
  - worker.resume_viral_video_pipeline
  - worker.run_video_style_analysis

apps/api/app/api/routes/viral_video.py

@@ -34,6 +34,7 @@ from packages.adapters.sqlalchemy_impl.viral_video_repository import (
     SQLAlchemyViralVideoJobRepository,
     SQLAlchemyViralVideoStyleTemplateRepository,
 )
+from app.core.celery_app import celery_app
 from packages.domain.viral_video import ViralVideoStatus
 
 logger = logging.getLogger(__name__)
@@ -122,9 +123,7 @@ def create_viral_video(
 
     # 入队 Celery 任务
     try:
-        from worker_app.tasks.viral_video import run_viral_video_pipeline
-
-        run_viral_video_pipeline.delay(job.id)
+        celery_app.send_task("worker.run_viral_video_pipeline", args=[job.id])
         logger.info("[爆款视频] 任务已入队: job_id=%s user_id=%s", job.id, job.user_id)
     except Exception as e:
         logger.error("[爆款视频] 入队失败: %s", e, exc_info=True)
@@ -210,9 +209,7 @@ def retry_viral_video_job(
 
     # 重新入队
     try:
-        from worker_app.tasks.viral_video import run_viral_video_pipeline
-
-        run_viral_video_pipeline.delay(job.id)
+        celery_app.send_task("worker.run_viral_video_pipeline", args=[job.id])
         logger.info("[爆款视频] 重试入队: job_id=%s retry_count=%d", job.id, job.retry_count)
     except Exception as e:
         logger.error("[爆款视频] 重试入队失败: %s", e, exc_info=True)
@@ -249,9 +246,7 @@ def confirm_intent(
 
     # 从断点恢复 Celery 任务
     try:
-        from worker_app.tasks.viral_video import resume_viral_video_pipeline
-
-        resume_viral_video_pipeline.delay(job.id)
+        celery_app.send_task("worker.resume_viral_video_pipeline", args=[job.id])
         logger.info("[爆款视频] 意图确认，恢复流水线: job_id=%s", job.id)
     except Exception as e:
         logger.error("[爆款视频] 恢复流水线失败: %s", e, exc_info=True)
@@ -284,9 +279,7 @@ def analyze_style(
 
     # 入队风格分析任务
     try:
-        from worker_app.tasks.viral_video import run_video_style_analysis
-
-        run_video_style_analysis.delay(job.id)
+        celery_app.send_task("worker.run_video_style_analysis", args=[job.id])
         logger.info("[爆款视频] 风格分析入队: job_id=%s", job.id)
     except Exception as e:
         logger.error("[爆款视频] 风格分析入队失败: %s", e, exc_info=True)