feat(auth): require a captcha on the endpoints that send mail (#32)

Signup had no bot protection. The account itself was never the prize — GoTrue
refuses a session until the address is confirmed, and 0 of the 2 unconfirmed
accounts on record ever signed in. The prize is our mail: /signup and /recover
send to whatever address the caller types, so a stranger could make
pezkuwichain.io mail thousands of people who never asked for it. Sending
reputation takes months to rebuild and every product on the domain shares it.

Rate limiting alone was not enough. GoTrue defaults to 30 emails/hour, which is
720 a day to attacker-chosen recipients; tightening it to 15 halves the number
without changing what is possible.

Verification runs inside GoTrue, not here. It POSTs to siteverify with the
widget secret before /signup, /recover, /resend, /magiclink, /otp and /token
with grant_type=password. Adding our own check in the frontend or a proxy would
protect nothing, because /auth/v1/signup is reachable directly.

One shared widget in execute mode rather than one per form: some of these are
buttons with nowhere to put a checkbox, and interaction-only appearance keeps
the challenge invisible unless Cloudflare asks for one. Tokens are single-use,
so each call renders fresh and tears down the previous widget.

Six call sites, matching the six protected endpoints — password login included,
since /token with grant_type=password is enforced. Refresh-token grants are
exempt, so open sessions are unaffected.

Site key travels as a build arg like the WalletConnect id: it is public by
construction, visible in the bundle. Only the secret is privileged and it lives
in GoTrue's env on the auth host.

Ships ahead of enforcement. Turning captcha on server-side before the frontend
sends tokens would break every login and signup in the same instant.

.github/workflows/quality-gate.yml

@@ -87,6 +87,7 @@ jobs:
           VITE_ASSET_HUB_ENDPOINT: wss://asset-hub-rpc.pezkuwichain.io
           VITE_PEOPLE_CHAIN_ENDPOINT: wss://people-rpc.pezkuwichain.io
           VITE_WALLETCONNECT_PROJECT_ID: 8292a793b7640e8364c378e331e76d04
+          VITE_TURNSTILE_SITE_KEY: 0x4AAAAAAEDkUkbsUgw2D9zN
 
       - name: Upload build artifact
         uses: actions/upload-artifact@v4
@@ -208,6 +209,7 @@ jobs:
             VITE_ASSET_HUB_ENDPOINT=wss://asset-hub-rpc.pezkuwichain.io
             VITE_PEOPLE_CHAIN_ENDPOINT=wss://people-rpc.pezkuwichain.io
             VITE_WALLETCONNECT_PROJECT_ID=8292a793b7640e8364c378e331e76d04
+            VITE_TURNSTILE_SITE_KEY=0x4AAAAAAEDkUkbsUgw2D9zN
             VITE_SUPABASE_URL=${{ secrets.VITE_SUPABASE_URL }}
             VITE_SUPABASE_ANON_KEY=${{ secrets.VITE_SUPABASE_ANON_KEY }}
           cache-from: type=registry,ref=${{ steps.meta.outputs.image }}:cache