feat(sub12b): fp.rack.tag — rack-label registry + 4 starter tags

M2M tag registry: Rush / Hold for QC / Damaged / Customer Sample.
Each rack can carry many tags; tags surface as coloured chips on the
plant-overview rack rows + Move Rack dialog (Task 13).

Plating → Configuration → Rack Tags menu (sequence 48).
post_init_hook seeds 4 starters — idempotent (no-op if any exist).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating/__init__.py

@@ -22,10 +22,12 @@ def post_init_hook(env):
        `kind` field.
     3. Sub 12a — seed fp.step.template with starter library entries
        derived from ENP-ALUM-BASIC if the library is currently empty.
+    4. Sub 12b — seed 4 starter rack tags if the registry is empty.
     """
     _seed_default_timezone(env)
     _backfill_node_input_kind(env)
     _seed_step_library_if_empty(env)
+    _seed_rack_tags_if_empty(env)
 
 
 def _seed_default_timezone(env):
@@ -185,3 +187,21 @@ def _seed_minimal_library(env):
         'Fusion Plating: seeded minimal step library (%s entries)',
         len(minimal),
     )
+
+
+def _seed_rack_tags_if_empty(env):
+    """Sub 12b — seed 4 starter rack tags."""
+    Tag = env['fp.rack.tag']
+    if Tag.search_count([]):
+        return
+    starters = [
+        ('Rush', 1),
+        ('Hold for QC', 3),
+        ('Damaged', 9),
+        ('Customer Sample', 5),
+    ]
+    for name, color in starters:
+        Tag.create({'name': name, 'color': color})
+    _logger.info(
+        'Fusion Plating: seeded %s starter rack tags', len(starters),
+    )