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/templates/job_detail.html

@@ -127,6 +127,34 @@
           <i class="bi bi-exclamation-triangle-fill me-2"></i>
           File <code>{{ job.file_name }}</code> not found on disk.
         </div>
+        {% elif analysis_failed %}
+        <div class="alert alert-warning py-2 px-3 mb-2" style="font-size:.875rem">
+          <i class="bi bi-exclamation-triangle me-2"></i>
+          Analysis failed or timed out — enter values from your slicer below.
+        </div>
+        <form method="POST" action="/jobs/{{ job.id }}/geometry" class="d-flex flex-column gap-2" style="font-size:.8rem">
+          <div class="d-flex gap-2 align-items-center">
+            <label class="text-secondary" style="width:70px">X mm</label>
+            <input type="number" step="0.1" name="cube_x" value="" required
+                   class="form-control form-control-sm bg-dark border-secondary text-white" style="width:100px">
+          </div>
+          <div class="d-flex gap-2 align-items-center">
+            <label class="text-secondary" style="width:70px">Y mm</label>
+            <input type="number" step="0.1" name="cube_y" value="" required
+                   class="form-control form-control-sm bg-dark border-secondary text-white" style="width:100px">
+          </div>
+          <div class="d-flex gap-2 align-items-center">
+            <label class="text-secondary" style="width:70px">Z mm</label>
+            <input type="number" step="0.1" name="cube_z" value="" required
+                   class="form-control form-control-sm bg-dark border-secondary text-white" style="width:100px">
+          </div>
+          <div class="d-flex gap-2 align-items-center">
+            <label class="text-secondary" style="width:70px">Vol cm³</label>
+            <input type="number" step="0.001" name="volume_ccm" value="" required
+                   class="form-control form-control-sm bg-dark border-secondary text-white" style="width:100px">
+          </div>
+          <div><button type="submit" class="btn btn-sm btn-outline-secondary py-0 px-2">Save</button></div>
+        </form>
         {% elif computing %}
         <div class="text-secondary small d-flex align-items-center gap-2" id="computing-state">
           <div class="spinner-border spinner-border-sm text-secondary" role="status"></div>