Add multi-user auth system and admin panel

- User model with email/hashed_password/is_admin/notification_prefs
- JWT auth: POST /api/auth/register, /login, /me
- First registered user auto-promoted to admin
- Migration 0005: users table + user_id FK on follows (clears global follows)
- Follows, dashboard, settings, admin endpoints all require authentication
- Admin endpoints (settings writes, celery triggers) require is_admin
- Frontend: login/register pages, Zustand auth store (localStorage persist)
- AuthGuard component gates all app routes, shows app shell only when authed
- Sidebar shows user email + logout; Admin nav link visible to admins only
- Admin panel (/settings): user list with delete + promote/demote, LLM config,
  data source settings, and manual celery controls

Authored-By: Jack Levy

frontend/app/register/page.tsx

@@ -0,0 +1,96 @@
+"use client";
+
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+import Link from "next/link";
+import { authAPI } from "@/lib/api";
+import { useAuthStore } from "@/stores/authStore";
+
+export default function RegisterPage() {
+  const router = useRouter();
+  const setAuth = useAuthStore((s) => s.setAuth);
+  const [email, setEmail] = useState("");
+  const [password, setPassword] = useState("");
+  const [error, setError] = useState("");
+  const [loading, setLoading] = useState(false);
+
+  async function handleSubmit(e: React.FormEvent) {
+    e.preventDefault();
+    setError("");
+    if (password.length < 8) {
+      setError("Password must be at least 8 characters.");
+      return;
+    }
+    setLoading(true);
+    try {
+      const { access_token, user } = await authAPI.register(email.trim(), password);
+      setAuth(access_token, { id: user.id, email: user.email, is_admin: user.is_admin });
+      router.replace("/");
+    } catch (err: unknown) {
+      const msg =
+        (err as { response?: { data?: { detail?: string } } })?.response?.data?.detail ||
+        "Registration failed. Please try again.";
+      setError(msg);
+    } finally {
+      setLoading(false);
+    }
+  }
+
+  return (
+    <div className="min-h-screen flex items-center justify-center bg-background">
+      <div className="w-full max-w-sm space-y-6 p-8 border rounded-lg bg-card shadow-sm">
+        <div>
+          <h1 className="text-2xl font-bold">PocketVeto</h1>
+          <p className="text-muted-foreground text-sm mt-1">Create your account</p>
+        </div>
+
+        <form onSubmit={handleSubmit} className="space-y-4">
+          <div className="space-y-1">
+            <label className="text-sm font-medium" htmlFor="email">Email</label>
+            <input
+              id="email"
+              type="email"
+              required
+              autoComplete="email"
+              value={email}
+              onChange={(e) => setEmail(e.target.value)}
+              className="w-full px-3 py-2 border rounded-md bg-background text-sm focus:outline-none focus:ring-2 focus:ring-ring"
+            />
+          </div>
+
+          <div className="space-y-1">
+            <label className="text-sm font-medium" htmlFor="password">
+              Password <span className="text-muted-foreground font-normal">(min 8 chars)</span>
+            </label>
+            <input
+              id="password"
+              type="password"
+              required
+              autoComplete="new-password"
+              value={password}
+              onChange={(e) => setPassword(e.target.value)}
+              className="w-full px-3 py-2 border rounded-md bg-background text-sm focus:outline-none focus:ring-2 focus:ring-ring"
+            />
+          </div>
+
+          {error && <p className="text-sm text-destructive">{error}</p>}
+
+          <button
+            type="submit"
+            disabled={loading}
+            className="w-full py-2 px-4 bg-primary text-primary-foreground rounded-md text-sm font-medium hover:bg-primary/90 disabled:opacity-50"
+          >
+            {loading ? "Creating account..." : "Create account"}
+          </button>
+        </form>
+
+        <p className="text-sm text-center text-muted-foreground">
+          Already have an account?{" "}
+          <Link href="/login" className="text-primary hover:underline">
+            Sign in
+          </Link>
+        </p>
+      </div>
+    </div>
+  );
+}