feat: per-line render overrides — override any output type setting at order time

Instead of duplicating output types for every variation (WebP vs PNG,
different resolution), keep one canonical output type and override
specific fields per order line via render_overrides JSONB.

Backend:
- render_overrides JSONB column on OrderLine (DB migration)
- Render task merges overrides with output type settings (format, width,
  height, samples, engine, denoiser, transparent_bg, cycles_device)
- POST /orders/{id}/batch-render-overrides endpoint for bulk override
- PatchLineBody accepts render_overrides for per-line patching

Frontend:
- Batch render overrides section on OrderDetail: output format dropdown
  (PNG/JPG/WebP) + resolution dropdown (512-4096)
- Clear button to remove overrides

MCP:
- create_order tool: accepts product_ids, output_type, render_overrides,
  material_override — enables "render all products as WebP" via Claude
- set_render_overrides tool: batch override on existing orders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/domains/orders/models.py

@@ -151,6 +151,7 @@ class OrderLine(Base):
         nullable=True,
     )
     material_override: Mapped[str | None] = mapped_column(String(200), nullable=True, default=None)
+    render_overrides: Mapped[dict | None] = mapped_column(JSONB, nullable=True, default=None)
     notes: Mapped[str | None] = mapped_column(Text, nullable=True)
     tenant_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True

backend/app/domains/orders/schemas.py

@@ -67,6 +67,7 @@ class OrderLineCreate(BaseModel):
     global_render_position_id: uuid.UUID | None = None
     gewuenschte_bildnummer: str | None = None
     material_override: str | None = None
+    render_overrides: dict | None = None
     notes: str | None = None
 
 
@@ -93,6 +94,7 @@ class OrderLineOut(BaseModel):
     render_started_at: datetime | None = None
     render_completed_at: datetime | None = None
     material_override: str | None = None
+    render_overrides: dict | None = None
     notes: str | None
     created_at: datetime
     updated_at: datetime