fix: 修复素材库诊断/缩略图/视频URL + 剪辑计划错误处理

Task #105: 修复剪辑计划/模板页面报服务器繁忙
- templates.py: list_templates/get_template/list_categories 加 try/except
- templates.py: 新增 POST /{template_id}/toggle-favorite 兼容端点
- edit_plans.py: list_plans 加 try/except，ai_tasks 加 ImportError 守卫

Task #106: 修复素材库诊断按钮 + 缩略图/视频URL
- asset_diagnosis.py: get_project_asset_diagnosis 加 try/except
- assets.py: 注入 storage_service，生成签名 file_url
- asset.py schema: 新增 file_url 字段
- 视频素材 thumbnail_url 为空时复用 file_url 作为封面

其他:
- edit_plans/generation_tasks 支持 source_edit_plan_id
- Alembic 迁移 022: 两表加 source_edit_plan_id 字段

packages/adapters/sqlalchemy_impl/models.py

@@ -119,6 +119,7 @@ class EditPlanModel(Base):
     status = Column(String(20), nullable=False, default="draft", index=True)
     total_duration = Column(Float, nullable=False, default=0.0)
     config = Column(JSON, nullable=False, default=dict)
+    source_edit_plan_id = Column(String(32), nullable=True, index=True)
     created_at = Column(DateTime, nullable=False, default=lambda: datetime.now(timezone.utc))
     updated_at = Column(DateTime, nullable=False, default=lambda: datetime.now(timezone.utc))
 
@@ -219,6 +220,7 @@ class GenerationTaskModel(Base):
     started_at = Column(DateTime, nullable=True)
     completed_at = Column(DateTime, nullable=True)
     created_by_user_id = Column(String(32), nullable=False, default="", index=True)
+    source_edit_plan_id = Column(String(32), nullable=True, index=True)
     extra_meta = Column("metadata", JSON, nullable=False, default=dict)
     created_at = Column(DateTime, nullable=False, default=lambda: datetime.now(timezone.utc))