Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/tests/auth.setup.ts

@@ -0,0 +1,13 @@
+import { test as setup } from "@playwright/test"
+import { firstSuperuser, firstSuperuserPassword } from "./config.ts"
+
+const authFile = "playwright/.auth/user.json"
+
+setup("authenticate", async ({ page }) => {
+  await page.goto("/login")
+  await page.getByTestId("email-input").fill(firstSuperuser)
+  await page.getByTestId("password-input").fill(firstSuperuserPassword)
+  await page.getByRole("button", { name: "Log In" }).click()
+  await page.waitForURL("/")
+  await page.context().storageState({ path: authFile })
+})