Add passkey (WebAuthn) authentication with self-registration

Enable users to sign up and sign in using passkeys (biometrics/security
keys) without needing a manually-created Authentik account. The existing
SSO login remains as an alternative.

Backend:
- Add WebAuthn registration/authentication endpoints via py-webauthn
- Issue HS256 JWTs for passkey users, with Redis-backed challenge storage
- Dual JWT verification in auth middleware (issuer-based routing: passkey
  HS256 vs Authentik RS256)
- PasskeyCredential model + migration making user.password nullable
- UserRepository with full CRUD for users and credentials

Frontend:
- AuthUser type abstraction unifying OIDC and passkey users
- Passkey service using @simplewebauthn/browser for WebAuthn ceremonies
- LoginModal redesigned with Sign In / Sign Up tabs
- Type migration from oidc-client-ts User to AuthUser across all services
  and components

crawler/frontend/src/components/ui/tabs.tsx

@@ -0,0 +1,60 @@
+import * as React from "react"
+import * as TabsPrimitive from "@radix-ui/react-tabs"
+
+import { cn } from "@/lib/utils"
+
+function Tabs({
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.Root>) {
+  return <TabsPrimitive.Root data-slot="tabs" {...props} />
+}
+
+function TabsList({
+  className,
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.List>) {
+  return (
+    <TabsPrimitive.List
+      data-slot="tabs-list"
+      className={cn(
+        "inline-flex h-10 items-center justify-center rounded-md bg-muted p-1 text-muted-foreground w-full",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function TabsTrigger({
+  className,
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.Trigger>) {
+  return (
+    <TabsPrimitive.Trigger
+      data-slot="tabs-trigger"
+      className={cn(
+        "inline-flex items-center justify-center whitespace-nowrap rounded-sm px-3 py-1.5 text-sm font-medium ring-offset-background transition-all focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:pointer-events-none disabled:opacity-50 data-[state=active]:bg-background data-[state=active]:text-foreground data-[state=active]:shadow-sm flex-1",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function TabsContent({
+  className,
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.Content>) {
+  return (
+    <TabsPrimitive.Content
+      data-slot="tabs-content"
+      className={cn(
+        "mt-2 ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+export { Tabs, TabsList, TabsTrigger, TabsContent }