feat: sharp edge pipeline V02, tessellation presets, media cache-bust, GMSH plan

Sharp Edge Pipeline V02:
- export_step_to_gltf.py: replace BRep_Tool.Polygon3D_s (returns None in XCAF) with
  GCPnts_UniformAbscissa curve sampling at 0.3mm step — extracts 17,129 segment pairs
- Inject sharp_edge_pairs + sharp_threshold_deg into GLB extras (scenes[0].extras)
  via binary GLB JSON-chunk patching (no extra dependency)
- export_gltf.py: read schaeffler_sharp_edge_pairs from Blender scene custom props,
  apply via KD-tree to mark edges sharp=True + seam=True (OCC mm Z-up → Blender transform)
- tools/restore_sharp_marks.py: dual-pass (dihedral angle + OCC pairs), updated coordinate
  transform (X, -Z, Y) * 0.001

Tessellation:
- Admin UI: Draft / Standard / Fine preset buttons with active-state highlighting
- Default angular deflection: preview 0.5→0.1 rad, production 0.2→0.05 rad
- export_glb.py: read updated defaults from system_settings

Media / Cache:
- media/service.py: get_download_url appends ?v={file_size_bytes} cache-buster
- media/router.py: Cache-Control: no-cache for all download/thumbnail endpoints

Render pipeline:
- still_render.py / turntable_render.py: shared GPU activation + camera improvements
- render_order_line.py: global render position support
- render_thumbnail.py: updated defaults

Frontend:
- InlineCadViewer: file_size_bytes-aware URL update triggers re-fetch on regeneration
- ThreeDViewer: material panel, part selection, PBR mode improvements
- Admin.tsx: tessellation preset cards, GMSH setting dropdown
- MediaBrowser, ProductDetail, OrderDetail, Orders: various UI improvements
- New: MaterialPanel, GlobalRenderPositionsPanel, StepIndicator components
- New: renderPositions.ts API client

Plans / Docs:
- plan.md: GMSH Frontal-Delaunay tessellation plan (6 tasks)
- LEARNINGS.md: OCC Polygon3D_s None issue + GCPnts fix
- .gitignore: add backend/core (core dump from root process)

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

backend/alembic/versions/055_global_render_positions.py

@@ -0,0 +1,32 @@
+"""Add global_render_positions table.
+
+Revision ID: 055
+Revises: 054
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID
+
+revision = "055"
+down_revision = "ce21c8a67543"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "global_render_positions",
+        sa.Column("id", UUID(as_uuid=True), primary_key=True, server_default=sa.text("gen_random_uuid()")),
+        sa.Column("name", sa.String(200), nullable=False),
+        sa.Column("rotation_x", sa.Float, nullable=False, server_default="0.0"),
+        sa.Column("rotation_y", sa.Float, nullable=False, server_default="0.0"),
+        sa.Column("rotation_z", sa.Float, nullable=False, server_default="0.0"),
+        sa.Column("is_default", sa.Boolean, nullable=False, server_default="false"),
+        sa.Column("sort_order", sa.Integer, nullable=False, server_default="0"),
+        sa.Column("created_at", sa.DateTime, nullable=False, server_default=sa.text("now()")),
+        sa.Column("updated_at", sa.DateTime, nullable=False, server_default=sa.text("now()")),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("global_render_positions")