feat(core): bind request services and implement registered tasks

Wire plugin services through Bind/InjectFrom and AppContext so HTTP and
workers resolve dependencies after Apply. Register TaskHandler objects
with persisted results, and implement send_email_code, mail:send,
cleanup_inactive_users, and dispatch_bot_msg.

backend/plugins/drivers/driver_inproc_cron/plugin.go

@@ -83,7 +83,11 @@ func (p *Plugin) Start(ctx context.Context) error {
 		p.scheduler = newInprocScheduler(p.coreCtx.Schedules(), p.coreCtx.Tasks(), taskSvc)
 	}
 
-	return p.scheduler.Start(ctx)
+	runCtx := ctx
+	if p.coreCtx != nil {
+		runCtx = core.WithAppContext(ctx, p.coreCtx.Root())
+	}
+	return p.scheduler.Start(runCtx)
 }
 
 // Stop terminates the in-process cron scheduler.