feat(notifications): register webhook + email channel types

Add WEBHOOK and EMAIL to the channel set, the dispatcher cascade, the
per-event default policy, the push-attempt ledger union, and the admin
diagnostic config-presence check. Channels are string-typed, so no
migration is needed. The dispatcher routes each new type through its
sender with the same decrypt + parse + classify path the existing
channels use; the cascade orders webhook + email between the self-host
relays and the Web Push fallback.

Add the webhook + email channel config shapes and their settings-form
validation schemas, plus the query-key entries for the two new config
reads.

src/app/api/admin/notifications/diagnostic/route.ts

@@ -134,6 +134,14 @@ function isChannelConfigPresent(
     if (type === "NTFY") {
       return typeof parsed.topic === "string" && parsed.topic.length > 0;
     }
+    if (type === "WEBHOOK") {
+      return typeof parsed.url === "string" && parsed.url.length > 0;
+    }
+    if (type === "EMAIL") {
+      return (
+        typeof parsed.recipient === "string" && parsed.recipient.length > 0
+      );
+    }
     return false;
   } catch {
     return false;