deck contribution sprint 1: TableSeat.deck_variant FK + select_role wiring — TDD

- epic.TableSeat gains deck_variant FK → DeckVariant (nullable, SET_NULL)
- select_role view assigns request.user.equipped_deck to seat on role confirmation
- Migration 0006_add_deck_variant_to_tableseat
- ITs: test_select_role_assigns_equipped_deck_to_seat,
       test_select_role_no_deck_leaves_deck_variant_null

Code architected by Disco DeDisco <discodedisco@outlook.com>
Git commit message Co-Authored-By:
Claude Sonnet 4.6 <noreply@anthropic.com>

src/apps/epic/migrations/0006_add_deck_variant_to_tableseat.py

@@ -0,0 +1,19 @@
+# Generated by Django 6.0 on 2026-04-28 02:35
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('epic', '0005_seed_astro_reference_tables'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='tableseat',
+            name='deck_variant',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='active_seats', to='epic.deckvariant'),
+        ),
+    ]