Isolate STL analysis in child process; fix 3MF memory usage

- _load_geometry(): compute extents/volume from individual Scene meshes
  without concatenating — avoids the RAM spike that crashed the container
- analyse_in_background(): spawns a child process (spawn context, not fork)
  with a 120s timeout; OOM/crash in child no longer takes down the app
- _failed set: tracks files that timed out or crashed; spinner stops and
  a warning with inline override form is shown instead of looping forever
- analysis_failed state passed to template; override form shown immediately
  when analysis failed (no expand needed)

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

app/routers/ui.py

@@ -26,7 +26,7 @@ from app.estimation import estimate  # also used in start_job
 from app.filamentdb import (available_colors, available_materials, find_matching_ams_roll,
                              get_stock, log_print_on_roll, spool_counts)
 from app.stl import get_stl_dir, list_stl_files
-from app.stl_analysis import analyse_in_background, get_cache_map, is_computing  # noqa: F401
+from app.stl_analysis import analyse_in_background, get_cache_map, is_computing, is_failed  # noqa: F401
 
 router = APIRouter()
 templates = Jinja2Templates(directory="app/templates")
@@ -276,9 +276,11 @@ def job_detail(job_id: int, request: Request, background_tasks: BackgroundTasks,
         else:
             stl = db.query(StlCache).filter_by(filename=job.file_name).first()
             if stl is None and not job.file_name.endswith(".gcode"):
-                if not is_computing(job.file_name):
+                if is_failed(job.file_name):
+                    file_missing = False  # file exists, analysis just failed
+                elif not is_computing(job.file_name):
                     background_tasks.add_task(analyse_in_background, job.file_name)
-                computing = True
+                computing = not is_failed(job.file_name)
 
     estimation = estimate(stl.volume_ccm, job.job_material) if stl and stl.volume_ccm else None
     return templates.TemplateResponse("job_detail.html", {
@@ -288,6 +290,7 @@ def job_detail(job_id: int, request: Request, background_tasks: BackgroundTasks,
         "stl": stl,
         "file_missing": file_missing,
         "computing": computing,
+        "analysis_failed": job.file_name and is_failed(job.file_name),
         "estimation": estimation,
         "printers": db.query(Printer).order_by(Printer.name).all(),
         "brackets": db.query(JobBracket).order_by(JobBracket.name).all(),