feat(core-shared): auth-gate mutating feature procedures

Adds a shared requireAuthenticated tRPC middleware (reads the server-
resolved ctx.user from createTrpcContext) and applies it to every
mutating feature procedure — blog.createArticle and media.deleteMedia
were anonymous-callable (audit finding B7). Read-only queries stay
public; features compose <x>ProtectedProcedure from their error-mapped
base procedure.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/core-shared/package.json

@@ -16,6 +16,7 @@
     "./payload": "./src/payload/index.ts",
     "./trpc/init": "./src/trpc/init.ts",
     "./trpc/context": "./src/trpc/context.ts",
+    "./trpc/require-authenticated": "./src/trpc/require-authenticated.ts",
     "./trpc/define-error-middleware": "./src/trpc/define-error-middleware.ts",
     "./instrumentation": "./src/instrumentation/index.ts",
     "./instrumentation/otel": "./src/instrumentation/otel/index.ts",