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/alembic/versions/f5906aaf75af_add_render_overrides_to_order_lines.py

@@ -0,0 +1,30 @@
+"""add render_overrides to order_lines
+
+Revision ID: f5906aaf75af
+Revises: 42b686e71c47
+Create Date: 2026-03-15 11:24:23.387869
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision: str = 'f5906aaf75af'
+down_revision: Union[str, None] = '42b686e71c47'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('order_lines', sa.Column('render_overrides', postgresql.JSONB(astext_type=sa.Text()), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('order_lines', 'render_overrides')
+    # ### end Alembic commands ###