feat(core): standalone image-gen endpoint + api.ts methods (profile/password/avatar/generate-image)

- POST /api/ai/generate-image/ — project-less image generation (AITask.project nullable, no schema change),
  reuses VolcanoArk image_generation + credit reserve/charge; modes image/model/cover.
  Verified: manage.py check clean; 2 active IMAGE models present (doubao-seedream-4.5/5.0).
  (Real generation calls Volcano API + charges credit — not yet live-tested to avoid spend.)
- api.ts: updateProfile / changePassword / uploadAvatar / generateImage

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/backend/apps/ai/urls.py

@@ -1,9 +1,12 @@
+from django.urls import path
 from rest_framework.routers import DefaultRouter
 
-from .views import AITaskViewSet, ModelConfigViewSet
+from .views import AITaskViewSet, GenerateImageView, ModelConfigViewSet
 
 router = DefaultRouter()
 router.register("tasks", AITaskViewSet, basename="ai-task")
 router.register("models", ModelConfigViewSet, basename="model-config")
 
-urlpatterns = router.urls
+urlpatterns = [
+    path("generate-image/", GenerateImageView.as_view(), name="ai-generate-image"),
+] + router.urls