feat(integrations): add dynamic OpenRouter model loading

- Remove hardcoded OPENROUTER_MODEL_CHOICES from openrouter.py
- Add API endpoint /integrations/openrouter/models/ to fetch models dynamically
- Models loaded from OpenRouter API with free models (':free') at top
- Update OpenRouterIntegration model_name field (remove choices, blank=True)
- Add async buildForm() with dynamic_choices support
- Show asterisks (********) for saved API keys with helpful placeholder

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

myproject/integrations/migrations/0010_alter_openrouterintegration_model_name.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.10 on 2026-01-23 15:04
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('integrations', '0009_alter_glmintegration_model_name_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='openrouterintegration',
+            name='model_name',
+            field=models.CharField(blank=True, default='', help_text='Название используемой модели OpenRouter (загружается автоматически)', max_length=200, verbose_name='Название модели'),
+        ),
+    ]