feat: Stage 2 — authentication and setup flow

Backend (tasks 2.1–2.6, 2.10):
- settings_repo: get/set/delete/get_all CRUD for the key-value settings table
- session_repo: create/get/delete/delete_expired for session rows
- setup_service: bcrypt password hashing, one-time-only enforcement,
  run_setup() / is_setup_complete() / get_password_hash()
- auth_service: login() with bcrypt verify + token creation,
  validate_session() with expiry check, logout()
- setup router: GET /api/setup (status), POST /api/setup (201 / 409)
- auth router: POST /api/auth/login (token + HttpOnly cookie),
               POST /api/auth/logout (clears cookie, idempotent)
- SetupRedirectMiddleware: 307 → /api/setup for all API paths until setup done
- require_auth dependency: cookie or Bearer token → Session or 401
- conftest.py: manually bootstraps app.state.db for router tests
  (ASGITransport does not trigger ASGI lifespan)
- 85 tests pass; ruff 0 errors; mypy --strict 0 errors

Frontend (tasks 2.7–2.9):
- types/auth.ts, types/setup.ts, api/auth.ts, api/setup.ts
- AuthProvider: sessionStorage-backed context (isAuthenticated, login, logout)
- RequireAuth: guard component → /login?next=<path> when unauthenticated
- SetupPage: Fluent UI form, client-side validation, inline errors
- LoginPage: single password input, ?next= redirect after success
- DashboardPage: placeholder (full impl Stage 5)
- App.tsx: full route tree (/setup, /login, /, *)

frontend/src/pages/DashboardPage.tsx

@@ -0,0 +1,30 @@
+/**
+ * Dashboard placeholder page.
+ *
+ * Full implementation is delivered in Stage 5.
+ */
+
+import { Text, makeStyles, tokens } from "@fluentui/react-components";
+
+const useStyles = makeStyles({
+  root: {
+    padding: tokens.spacingVerticalXXL,
+  },
+});
+
+/**
+ * Temporary dashboard placeholder rendered until Stage 5 is complete.
+ */
+export function DashboardPage(): JSX.Element {
+  const styles = useStyles();
+  return (
+    <div className={styles.root}>
+      <Text as="h1" size={700} weight="semibold">
+        Dashboard
+      </Text>
+      <Text as="p" size={300}>
+        Ban overview will be implemented in Stage 5.
+      </Text>
+    </div>
+  );
+}