feat: Better Auth OAuth/OIDC server for the Intervals.icu MCP product

DCR-capable (RFC 7591) OAuth authorization server so Claude's MCP connector
self-registers — the thing Authentik can't do until 2026.8.0. Users sign in with
Google/Apple; oidcProvider + jwt issue asymmetric JWTs (JWKS) the MCP server
verifies. Shares the existing CNPG Postgres (own `betterauth` schema).

Scaffold: auth config, minimal Node server + login page, Dockerfile, CI.

tsconfig.json

@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "outDir": "dist",
+    "rootDir": "src",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "declaration": false,
+    "sourceMap": false,
+    "types": ["node"]
+  },
+  "include": ["src/**/*.ts"]
+}