feat(C1+C2): workflow system — WorkflowDefinition + Celery Canvas builder

Migrations 037 (workflow tables + 3 seed definitions) + 038 (output_types.workflow_definition_id).
WorkflowDefinition/Run/NodeResult SQLAlchemy models in domains/rendering/models.py.
workflow_builder.py: dispatch_workflow() with Celery Canvas for still/turntable/multi_angle.
workflow_router.py: CRUD endpoints at /api/workflows (admin/PM guards).
dispatch_service.py: dispatch_render_with_workflow() prefers workflow path when
  OutputType.workflow_definition_id is set, falls back to legacy dispatch otherwise.
main.py: registers workflows_router.
models/__init__.py: re-exports WorkflowDefinition, WorkflowRun, WorkflowNodeResult.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/rendering/models.py

@@ -35,6 +35,10 @@ class OutputType(Base):
         DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False
     )
 
+    workflow_definition_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("workflow_definitions.id", ondelete="SET NULL"), nullable=True
+    )
+
     order_lines: Mapped[list["OrderLine"]] = relationship("OrderLine", back_populates="output_type")
     pricing_tier: Mapped["PricingTier | None"] = relationship("PricingTier", back_populates="output_types")
 
@@ -85,3 +89,62 @@ class ProductRenderPosition(Base):
 
     product: Mapped["Product"] = relationship("Product", back_populates="render_positions")
     order_lines: Mapped[list["OrderLine"]] = relationship("OrderLine", back_populates="render_position")
+
+
+class WorkflowDefinition(Base):
+    __tablename__ = "workflow_definitions"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name: Mapped[str] = mapped_column(String(200), nullable=False)
+    output_type_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("output_types.id", ondelete="SET NULL"), nullable=True
+    )
+    config: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+
+    runs: Mapped[list["WorkflowRun"]] = relationship(
+        "WorkflowRun", back_populates="workflow_def", lazy="noload", cascade="all, delete-orphan"
+    )
+
+
+class WorkflowRun(Base):
+    __tablename__ = "workflow_runs"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    workflow_def_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("workflow_definitions.id", ondelete="SET NULL"), nullable=True
+    )
+    order_line_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("order_lines.id", ondelete="CASCADE"), nullable=True, index=True
+    )
+    celery_task_id: Mapped[str | None] = mapped_column(String(500), nullable=True)
+    status: Mapped[str] = mapped_column(String(50), nullable=False, default="pending")
+    started_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
+    completed_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
+    error_message: Mapped[str | None] = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+
+    workflow_def: Mapped["WorkflowDefinition | None"] = relationship(
+        "WorkflowDefinition", back_populates="runs"
+    )
+    node_results: Mapped[list["WorkflowNodeResult"]] = relationship(
+        "WorkflowNodeResult", back_populates="run", cascade="all, delete-orphan"
+    )
+
+
+class WorkflowNodeResult(Base):
+    __tablename__ = "workflow_node_results"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    run_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("workflow_runs.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    node_name: Mapped[str] = mapped_column(String(200), nullable=False)
+    status: Mapped[str] = mapped_column(String(50), nullable=False, default="pending")
+    output: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
+    log: Mapped[str | None] = mapped_column(Text, nullable=True)
+    duration_s: Mapped[float | None] = mapped_column(Float, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+
+    run: Mapped["WorkflowRun"] = relationship("WorkflowRun", back_populates="node_results")