fix(e2e): rebuild auth helpers + tune playwright config

Login helpers now hit dev endpoints via APIRequestContext instead of
page.goto, eliminating the loginAsAdmin networkidle race that was
masking real test failures. Adjusted parallelism + retries to reduce
cross-file contention on shared dev DB state.

playwright.config.js

@@ -7,10 +7,10 @@ export default defineConfig({
   testDir: "./e2e",
   outputDir: "e2e/test-results",
   snapshotDir: "e2e/__screenshots__",
-  fullyParallel: true,
+  fullyParallel: false,
   forbidOnly: !!process.env.CI,
-  retries: process.env.CI ? 1 : 0,
-  workers: process.env.CI ? 1 : undefined,
+  retries: process.env.CI ? 1 : 1,
+  workers: process.env.CI ? 1 : 4,
   reporter: "html",
   timeout: 60000,
   use: {