fix(jobs): map fp.coating.config.thickness_uom to fp.job.step.thickness_uom

The recipe→steps generator was copying coating.thickness_uom blind
into fp.job.step.thickness_uom, but the two selections use
different value codes:

  fp.coating.config.thickness_uom : 'mils' / 'microns' / 'inches'
  fp.job.step.thickness_uom       : 'mil'  / 'um'      / 'inch'

Result: any SO confirmed with a coating using the long-form codes
(real demo data uses 'mils') hit a 'Wrong value for ...' selection
error, the savepoint rolled back, and the fp.job ended up with 0
steps.

Add an explicit mapping. Unknown values fall through to the step
default ('um'). Demo seed re-run after the fix produces 234 steps
across 31 jobs (was 207); thickness_uom distribution: 228 um, 6 mil.

Part of: native job model migration (spec 2026-04-25)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_jobs/models/fp_job.py

@@ -224,7 +224,24 @@ class FpJob(models.Model):
                             'thickness_uom' in coating._fields
                             and coating.thickness_uom
                         ):
-                            vals['thickness_uom'] = coating.thickness_uom
+                            # fp.coating.config uses long-form uom names
+                            # (mils / microns / inches); fp.job.step uses
+                            # short codes (mil / um / inch). Map between
+                            # them. Unknown values fall through to the
+                            # step's default ('um').
+                            _UOM_MAP = {
+                                'mils': 'mil',
+                                'mil': 'mil',
+                                'microns': 'um',
+                                'micron': 'um',
+                                'um': 'um',
+                                'inches': 'inch',
+                                'inch': 'inch',
+                                'in': 'inch',
+                            }
+                            mapped = _UOM_MAP.get(coating.thickness_uom)
+                            if mapped:
+                                vals['thickness_uom'] = mapped
 
                     step_vals_list.append(vals)
                     if instructions: