Add comprehensive backend test suite (#339)

* add test suite, mostly covers integration testing, tests are only backend side

* workflow runs the correct script

* workflow runs the correct script

* workflow runs the correct script

* unit tests incoming

* Fix multer silent rejections and error handler info leak

- Revert cb(null, false) to cb(new Error(...)) in auth.ts, collab.ts,
  and files.ts so invalid uploads return an error instead of silently
  dropping the file
- Error handler in app.ts now always returns 500 / "Internal server
  error" instead of forwarding err.message to the client

* Use statusCode consistently for multer errors and error handler

- Error handler in app.ts reads err.statusCode to forward the correct
  HTTP status while keeping the response body generic

server/vitest.config.ts

@@ -0,0 +1,33 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    root: '.',
+    include: ['tests/**/*.test.ts'],
+    globals: true,
+    setupFiles: ['tests/setup.ts'],
+    testTimeout: 15000,
+    hookTimeout: 15000,
+    pool: 'forks',
+    silent: false,
+    reporters: ['verbose'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['lcov', 'text'],
+      reportsDirectory: './coverage',
+      include: ['src/**/*.ts'],
+    },
+  },
+  resolve: {
+    alias: {
+      '@modelcontextprotocol/sdk/server/mcp': new URL(
+          './node_modules/@modelcontextprotocol/sdk/dist/cjs/server/mcp.js',
+          import.meta.url
+      ).pathname,
+      '@modelcontextprotocol/sdk/server/streamableHttp': new URL(
+          './node_modules/@modelcontextprotocol/sdk/dist/cjs/server/streamableHttp.js',
+          import.meta.url
+      ).pathname,
+    },
+  },
+});