Phase 2a — add 'http' handler (sync forwarder)

The http handler POSTs the Telegram Update JSON to a configurable
internal URL and expects a JSON {text} reply, which it sends back via
sendMessage. Sync : the webhook ack waits for the upstream answer
(timeout default 5s, capped at 30s — Telegram itself closes around 60s).

For slow / unreliable backends use the Phase 3 async handlers once the
queue is in place.

YAML config :

  bots:
    webappbot:
      handler: http
      http:
        url: http://webapp.webapp.svc.cluster.local:8080/telegram/update
        timeout: 5s

Refs ~/.claude/plans/pour-les-notifications-on-inherited-seal.md § Phase 2.

handlers.go

@@ -73,6 +73,15 @@ func NewRegistry(cfg *Config, tg *TelegramClient, auth *Auth) (*Registry, error)
 				return nil, fmt.Errorf("bot %s uses handler=auth but AUTH_SECRET is unset", slug)
 			}
 			h = &AuthHandler{tg: tg, auth: auth}
+		case "http":
+			if b.HTTP == nil {
+				return nil, fmt.Errorf("bot %s uses handler=http but no `http:` config block was provided", slug)
+			}
+			hh, err := NewHTTPHandler(tg, *b.HTTP)
+			if err != nil {
+				return nil, fmt.Errorf("bot %s: %w", slug, err)
+			}
+			h = hh
 		case "":
 			return nil, fmt.Errorf("bot %s: handler missing", slug)
 		default: