add a fallback proxy config

cmd/cocoon/main.go

···
 				EnvVars: []string{"COCOON_BLOCKSTORE_VARIANT"},
 				Value:   "sqlite",
 			},
+			&cli.StringFlag{
+				Name:    "fallback-proxy",
+				EnvVars: []string{"COCOON_FALLBACK_PROXY"},
+			},
 		},
 		Commands: []*cli.Command{
 			runServe,
···
 			},
 			SessionSecret:     cmd.String("session-secret"),
 			BlockstoreVariant: server.MustReturnBlockstoreVariant(cmd.String("blockstore-variant")),
+			FallbackProxy:     cmd.String("fallback-proxy"),
 		})
 		if err != nil {
 			fmt.Printf("error creating cocoon: %v", err)

server/handle_proxy.go

···
 
 func (s *Server) getAtprotoProxyEndpointFromRequest(e echo.Context) (string, string, error) {
 	svc := e.Request().Header.Get("atproto-proxy")
+	if svc == "" && s.config.FallbackProxy != "" {
+		svc = s.config.FallbackProxy
+	}
 
 	svcPts := strings.Split(svc, "#")
 	if len(svcPts) != 2 {

server/server.go

···
 	oauthProvider *provider.Provider
 	evtman        *events.EventManager
 	passport      *identity.Passport
+	fallbackProxy string
 
 	dbName   string
 	s3Config *S3Config
···
 	SmtpEmail         string
 	SmtpName          string
 	BlockstoreVariant BlockstoreVariant
+	FallbackProxy     string
 }
 
 type CustomValidator struct {
···
 			SmtpName:          args.SmtpName,
 			SmtpEmail:         args.SmtpEmail,
 			BlockstoreVariant: args.BlockstoreVariant,
+			FallbackProxy:     args.FallbackProxy,
 		},
 		evtman:   events.NewEventManager(events.NewMemPersister()),
 		passport: identity.NewPassport(h, identity.NewMemCache(10_000)),