refactor(P1): complete pipeline cleanup — M1 dead code + M3 blender split

M1 dead code removal:
- admin.py: remove VALID_STL_QUALITIES + stl_quality (7 locations)
- frontend: remove stl_quality from 6 files (api/orders.ts, api/worker.ts,
  WorkerActivity.tsx, RenderInfoModal.tsx, helpTexts.ts, mocks/handlers.ts)
- blender_render.py: delete _mark_sharp_and_seams() — dead, never called (62 lines)
- step_processor.py: delete _render_via_service() + 2 elif renderer=="threejs" branches
- renderproblems_tmp/: remove 3 orphaned debug images

M3 blender_render.py decomposition (858 → 248 lines):
- _blender_gpu.py: activate_gpu(), configure_engine()
- _blender_import.py: import_glb(), apply_rotation()
- _blender_materials.py: FAILED_MATERIAL_NAME, assign_failed_material(),
  build_mat_map_lower(), apply_material_library()
- _blender_camera.py: setup_auto_camera(), setup_auto_lights()
- _blender_scene.py: ensure_collection(), apply_smooth_batch(),
  apply_sharp_edges_from_occ(), setup_shadow_catcher()
- Entry-point: sys.path.insert for submodule discovery; arg-parse + Mode A/B orchestration only

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

frontend/src/__tests__/mocks/handlers.ts

@@ -7,7 +7,6 @@ export const handlers = [
       blender_cycles_samples: 256,
       blender_eevee_samples: 64,
       thumbnail_format: 'jpg',
-      stl_quality: 'low',
       blender_smooth_angle: 30,
       cycles_device: 'auto',
       blender_max_concurrent_renders: 3,

frontend/src/api/orders.ts

@@ -9,7 +9,6 @@ export interface RenderLog {
   engine?: string
   engine_used?: string
   samples?: number
-  stl_quality?: string
   smooth_angle?: number
   total_duration_s?: number
   stl_duration_s?: number

frontend/src/api/worker.ts

@@ -7,7 +7,6 @@ export interface RenderLog {
   engine_used?: string
   samples?: number
   cycles_device?: string
-  stl_quality?: string
   smooth_angle?: number
   width?: number
   height?: number

frontend/src/components/renders/RenderInfoModal.tsx

@@ -141,7 +141,6 @@ export default function RenderInfoModal({
                 )}
                 {rl.format && <Row label="Format" value={rl.format.toUpperCase()} />}
                 {rl.parts_count != null && <Row label="Parts" value={rl.parts_count} />}
-                {rl.stl_quality && <Row label="STL Quality" value={rl.stl_quality} />}
                 {rl.smooth_angle != null && <Row label="Smooth Angle" value={`${rl.smooth_angle}°`} />}
               </div>
             </div>

frontend/src/help/helpTexts.ts

@@ -35,11 +35,6 @@ export const HELP_TEXTS: Record<string, HelpText> = {
     body: 'Which renderer to use for STEP file thumbnails. Blender produces photorealistic results; Three.js is faster but lower quality.',
     recommendation: 'Use Blender for production, Three.js for fast previews',
   },
-  'setting.stl_quality': {
-    title: 'STL Export Quality',
-    body: 'Controls tessellation precision when converting STEP to STL for older render paths. High quality = finer mesh, larger file, slower conversion.',
-    recommendation: 'Low is sufficient for most thumbnails',
-  },
   'action.regenerate_thumbnails': {
     title: 'Regenerate All Thumbnails',
     body: 'Re-renders thumbnails for all STEP files using current renderer settings. Queues every file on the thumbnail_rendering worker.',

frontend/src/pages/WorkerActivity.tsx

@@ -524,7 +524,6 @@ function RenderDetails({ entry }: { entry: CadActivityEntry }) {
               <KV label="Engine" value={log.engine_used ?? log.engine ?? '—'} highlight={log.engine_used !== log.engine} />
               <KV label="Samples" value={log.samples?.toString() ?? '—'} />
               <KV label="Device" value={log.cycles_device ?? '—'} />
-              <KV label="STL quality" value={log.stl_quality ?? '—'} />
               <KV label="Smooth angle" value={log.smooth_angle != null ? `${log.smooth_angle}°` : '—'} />
               <KV label="Resolution" value={log.width && log.height ? `${log.width}×${log.height}` : '—'} />
             </>}