feat: complete remaining Go backend modules

contracts/usage/http-v1.json

@@ -0,0 +1,11 @@
+{
+  "version": 1,
+  "routes": [
+    { "method": "GET", "path": "/api/usage", "requirement": "app", "accountSource": "refreshed_session", "defaultPreset": "month" },
+    { "method": "GET", "path": "/api/admin/usage", "requirement": "admin", "organizationAdminScope": "refreshed_session", "organizationAdminRedactions": ["accounts", "recent", "options.accounts"] }
+  ],
+  "presets": ["today", "7d", "30d", "month"],
+  "capabilities": ["image.generate", "video.generate"],
+  "providers": ["volcengine-visual", "evolink", "seedance", "bailian"],
+  "infrastructureStatus": 500
+}