Add fairy feature flags and invite system:
- Feature flag system (fairies_enabled, fairies_purchase_enabled) with
admin UI
  - Update pricing page with Paddle payment integration
- Fairy invite flow should work for signed out users, redirects them
after sign in
  - Redirect to home when fairy flags disabled
  - Admin page for toggling feature flags
  - Extract payment logic from sidebar to reusable usePaddle hook
  - Add descriptions and reddemedby to fairy invites.

### Change type

- [x] `other`



<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds KV-backed feature flags (fairies, fairies_purchase) with admin
controls, gates UI/DO/routes, enhances fairy invites
(description/redeemedBy, signed-out flow), and centralizes Paddle
checkout used on pricing and sidebar.
> 
> - **Feature Flags (global)**:
> - Add KV-backed flags `fairies` and `fairies_purchase` with
`/app/feature-flags` API and polling `FeatureFlagsFetcher` +
`useFeatureFlags`.
> - Gate client hooks (`useFairyAccess`), UI (sidebar/purchase/pricing),
and DO `/fairy/has-access` + invite redemption by flags.
> - **Checkout / Pricing**:
> - Extract Paddle integration to `usePaddle` and use in pricing page
and sidebar link.
> - Pricing page: sign-in redirect (`redirect-to`), conditional button
labels, deep-link checkout via `?checkout=true`.
> - **Fairy Invites**:
> - Add `description` and `redeemedBy` tracking (DB migration) and show
in Admin table; include description on create.
> - Signed-out invite flow stores token and triggers sign-in, then
processes via root handler.
> - **Admin**:
> - New Feature Flags section (toggle flags) and updated Fairy Invites
management (grant/remove access unified, description input, copy link).
> - **Backend/Worker**:
> - New feature flag utils (get/set, defaults by env), admin routes for
flags, worker route exposure, KV bindings in wrangler.
> - **i18n/UI**:
> - Add strings for “Home”, “Start your fairy adventure”, “Your fairies
are waiting for you! →”.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
bc78b6f. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->