fix: add foreign key constraints to all ORM models

- Add 35 ForeignKey constraints across 15 tables (was zero)
- ON DELETE CASCADE for parent-child relationships (project → children)
- ON DELETE SET NULL for optional user references (created_by, assignee, etc.)
- Add relationship() with back_populates and cascade on all models
- Alter 11 columns from nullable=False to nullable=True for SET NULL
- Add 2 missing indexes on FK columns (assets.uploaded_by_user_id,
  project_titles.created_by_user_id)
- Update Pydantic response models to accept str | None for nullable fields
- Add Alembic migration 010 with complete upgrade/downgrade

Closes #17

apps/api/app/api/routes/project_management.py

@@ -65,8 +65,8 @@ class TaskResponse(BaseModel):
     description: str
     status: TaskStatus
     priority: TaskPriority
-    parent_task_id: str
-    assignee_user_id: str
+    parent_task_id: str | None
+    assignee_user_id: str | None
     progress: float
     planned_start_date: datetime | None
     planned_end_date: datetime | None
@@ -126,7 +126,7 @@ class TaskIssueResponse(BaseModel):
     description: str
     resolved: bool
     resolved_at: datetime | None
-    created_by_user_id: str
+    created_by_user_id: str | None
     created_at: datetime
     updated_at: datetime
 

apps/api/app/schemas/edit_plan.py

@@ -30,7 +30,7 @@ class EditPlanResponse(BaseModel):
     project_id: str
     template_id: str
     asset_library_id: str
-    title_id: str = ""
+    title_id: str | None = ""
     status: str
     summary: str
     editing_mode: str | None = None  # one_take, pip, voice_over, voice_pip

apps/api/app/schemas/generation_task.py

@@ -16,7 +16,7 @@ class GenerationTaskResponse(BaseModel):
     asset_library_id: str
     strategy_id: str
     voice_library_id: str
-    edit_plan_id: str
+    edit_plan_id: str | None
     status: str
     progress: float
     result_count: int

apps/api/app/schemas/ingest_job.py

@@ -14,4 +14,4 @@ class IngestJobResponse(BaseModel):
     storage_key: str
     status: str
     error_message: str
-    result_asset_id: str
+    result_asset_id: str | None