Add admin web UI (Phase 2): spend dashboard, media gallery, provider management

Fastify + single-page frontend on shared @nexusai/core. Reads the same SQLite
journal and media dir. Features: usage stats (by provider/capability/day),
content gallery (image/audio/video with local file serving + path-traversal
guard), provider CRUD with encrypted keys (masked in UI), enable/disable,
set-default, and real key validation via zero-cost /media/models call.
Adds Journal.getById and NordRouterMediaProvider.checkKey to core.

NexusAI/packages/admin/tsconfig.json

@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "CommonJS",
+    "moduleResolution": "node",
+    "declaration": false,
+    "sourceMap": true,
+    "outDir": "dist",
+    "rootDir": "src",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "resolveJsonModule": true
+  },
+  "include": ["src/**/*"]
+}