feat(admin): delete a whitelist entry, and stop hiding a broken mail setup

Three operator gaps, all found the same way — a person could not sign up and the
app showed nobody anything.

- DELETE /api/admin/invites/{id} with a delete control on both the pending
  requests and the previously-decided list. Deny keeps the row so the person
  cannot immediately re-ask; delete forgets the address. Refused with 409 while
  a User row exists for it, because is_allowed reads that row and removing it
  would revoke a live user's access with no other sign — the list ships
  has_account so the UI withholds the button instead of offering one that fails.
- Email is fail-soft by design, which had come to mean invisible. _send() now
  records an email_undelivered notification for every admin when a message is
  skipped or rejected, naming the message that was lost, throttled to one an
  hour. /api/me carries email_configured, driving a standing warning on
  Configuration → Email / SMTP and above the access-request list.
- The sign-in card's Create account / Forgot password / Didn't get the email
  views are history entries (useHistorySubview), so Back returns to the previous
  view instead of only moving the address bar.

Also: the per-test rate-limiter reset sweeps every loaded app.* module rather
than a hand-kept list of three — this change added a limiter outside it.

backend/app/routes/me.py

@@ -13,6 +13,7 @@ from app.auth import (
     optional_current_user,
     purge_user,
 )
+from app import email as email_mod
 from app import sysconfig
 from app.db import get_db
 from app.models import Invite, User
@@ -98,11 +99,16 @@ def get_me(
     if user is None:
         return {"authenticated": False}
     pending_invites = 0
+    # Admin-only, because it drives an operator warning: email is fail-soft, so a half-filled SMTP
+    # config costs verification links, password resets and access-request notices with nothing
+    # visible anywhere. Surfacing it here puts the warning on the screens where onboarding happens.
+    email_configured = True
     if user.role == "admin":
         pending_invites = (
             db.scalar(select(func.count()).select_from(Invite).where(Invite.status == "pending"))
             or 0
         )
+        email_configured = email_mod.email_enabled()
     return {
         "authenticated": True,
         "id": user.id,
@@ -121,6 +127,7 @@ def get_me(
         "can_read": has_read_scope(user),
         "can_write": has_write_scope(user),
         "pending_invites": pending_invites,
+        "email_configured": email_configured,
         "preferences": user.preferences or {},
     }