Initial commit: Pixel AI comic/video creation platform

- FastAPI backend with SQLModel, Alembic migrations, AgentScope agents
- Next.js 15 frontend with React 19, Tailwind, Zustand, React Flow
- Multi-provider AI system (DashScope, Kling, MiniMax, Volcengine, OpenAI, etc.)
- All HTTP clients migrated from sync requests to async httpx
- Admin-managed API keys via environment variables
- SSRF vulnerability fixed in ensure_url()

backend/src/config/services/provider.json.example

@@ -0,0 +1,7 @@
+{
+  "id": "provider-id",
+  "name": "Provider Display Name",
+  "description": "Optional description",
+  "api_key": "YOUR_API_KEY_OR_SET_VIA_ENV",
+  "dashboard_url": "https://example.com/dashboard"
+}