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/tests/conftest.py

@@ -196,13 +196,17 @@ def _reset_rate_limiters():
     """Every RateLimiter is a module-level singleton keyed by client IP, and TestClient always
     presents the same host — so without this the whole suite's login/register attempts pool into one
     300s window and, past the cap, some unrelated test starts failing with a 429 depending on order.
-    Cleared before each test; a test that wants to exercise throttling can still exhaust one itself."""
-    import app.routes.auth as _auth
-    import app.routes.messages as _messages
-    import app.routes.public as _public
+    Cleared before each test; a test that wants to exercise throttling can still exhaust one itself.
+
+    Sweeps every loaded `app.*` module rather than a hand-kept list, so a limiter added in a new
+    module is covered the day it lands instead of silently pooling until something fails."""
+    import sys
+
     from app.ratelimit import RateLimiter
 
-    for mod in (_auth, _messages, _public):
+    for name, mod in list(sys.modules.items()):
+        if name != "app" and not name.startswith("app."):
+            continue
         for obj in vars(mod).values():
             if isinstance(obj, RateLimiter):
                 obj.reset()