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>

frontend/src/components/shared/StepIndicator.tsx

@@ -0,0 +1,66 @@
+import { CheckCircle2 } from 'lucide-react'
+
+interface StepIndicatorProps {
+  step: number      // current step (1-based)
+  total: number
+  labels: string[]
+}
+
+export default function StepIndicator({ step, total, labels }: StepIndicatorProps) {
+  return (
+    <>
+      {/* Mobile: simple text */}
+      <div className="md:hidden flex items-center justify-center py-3">
+        <span className="text-sm font-medium text-content-secondary">
+          Step {step} of {total}
+          {labels[step - 1] ? ` — ${labels[step - 1]}` : ''}
+        </span>
+      </div>
+
+      {/* Desktop: full step bar */}
+      <div className="hidden md:flex items-center w-full mb-6">
+        {Array.from({ length: total }, (_, i) => {
+          const num = i + 1
+          const isCompleted = num < step
+          const isActive = num === step
+          const isFuture = num > step
+
+          return (
+            <div key={num} className="flex items-center flex-1 last:flex-none">
+              {/* Step circle + label */}
+              <div className="flex flex-col items-center gap-1">
+                <div
+                  className={`w-8 h-8 rounded-full flex items-center justify-center text-sm font-semibold transition-colors ${
+                    isCompleted
+                      ? 'bg-accent text-accent-text'
+                      : isActive
+                        ? 'bg-accent text-accent-text ring-4 ring-accent-light'
+                        : 'bg-surface-muted text-content-muted border border-border-default'
+                  }`}
+                >
+                  {isCompleted ? <CheckCircle2 size={16} /> : num}
+                </div>
+                <span
+                  className={`text-xs font-medium whitespace-nowrap ${
+                    isActive ? 'text-accent' : isFuture ? 'text-content-muted' : 'text-content-secondary'
+                  }`}
+                >
+                  {labels[i] ?? `Step ${num}`}
+                </span>
+              </div>
+
+              {/* Connector line (not after last step) */}
+              {num < total && (
+                <div
+                  className={`flex-1 h-0.5 mx-2 mt-[-16px] transition-colors ${
+                    isCompleted ? 'bg-accent' : 'bg-border-default'
+                  }`}
+                />
+              )}
+            </div>
+          )
+        })}
+      </div>
+    </>
+  )
+}