feat(notify): per-channel delivery results + accurate notification metrics

Dispatcher.Send now returns []ChannelResult{Type, Err} alongside the
aggregated error, and scheduler.checkDomain increments
NotificationsTotal per channel type/status instead of a single
unconditional IncNotification("dispatch", newStatus) placeholder that
ignored per-channel delivery outcome.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

internal/notify/dispatch.go

@@ -49,14 +49,23 @@ func NewDispatcher(store ChannelStore, cipher Decryptor) *Dispatcher {
 	}
 }
 
+// ChannelResult is the per-channel delivery outcome, so callers can record
+// success/failure metrics per channel type instead of one aggregate blob.
+type ChannelResult struct {
+	Type string
+	Err  error
+}
+
 // Send delivers ev to every enabled channel of projectID. Errors from
 // individual channels are aggregated (via errors.Join) rather than aborting
-// delivery to the remaining channels.
-func (d *Dispatcher) Send(ctx context.Context, projectID uuid.UUID, ev Event) error {
+// delivery to the remaining channels; the per-channel outcome is also
+// returned so callers can record accurate per-channel/status metrics.
+func (d *Dispatcher) Send(ctx context.Context, projectID uuid.UUID, ev Event) ([]ChannelResult, error) {
 	channels, err := d.store.ListEnabledChannels(ctx, projectID)
 	if err != nil {
-		return err
+		return nil, err
 	}
+	var results []ChannelResult
 	var errs []error
 	for _, ch := range channels {
 		n, ok := d.byType[ch.Type]
@@ -65,18 +74,21 @@ func (d *Dispatcher) Send(ctx context.Context, projectID uuid.UUID, ev Event) er
 		}
 		secret := ""
 		if ch.SecretEnc != "" {
-			b, err := d.cipher.Decrypt(ch.SecretEnc)
-			if err != nil {
-				errs = append(errs, err)
+			b, derr := d.cipher.Decrypt(ch.SecretEnc)
+			if derr != nil {
+				errs = append(errs, derr)
+				results = append(results, ChannelResult{Type: ch.Type, Err: derr})
 				continue
 			}
 			secret = string(b)
 		}
-		if err := n.Send(ctx, ch.Config, secret, ev); err != nil {
-			errs = append(errs, err)
+		serr := n.Send(ctx, ch.Config, secret, ev)
+		if serr != nil {
+			errs = append(errs, serr)
 		}
+		results = append(results, ChannelResult{Type: ch.Type, Err: serr})
 	}
-	return errors.Join(errs...)
+	return results, errors.Join(errs...)
 }
 
 // SendTest sends a single synthetic Event directly through the Notifier for