Drop the local Postgres/Zitadel/SeaweedFS stack, use shared Infrastructure

This repo ran its own separate copy of Postgres, Zitadel and SeaweedFS,
none of which are actually wired into the Go backend yet (auth and file
storage are still unchecked TODOs in the README). Removed that unused
duplicate stack (deploy/docker-compose.dev.yml, deploy/postgres-init/)
in favor of the shared Infrastructure repo also used by Musify and Ping.

make dev-infra now brings up Infrastructure instead, and backend/.env.example
points at its ports (Postgres on dockermanager_db, Zitadel, SeaweedFS)
as placeholders for when those features are implemented.

Makefile

@@ -1,10 +1,14 @@
 .PHONY: dev-infra dev-infra-down backend frontend migrate-up migrate-down lint test
 
+# Postgres, Zitadel and SeaweedFS are not run by this repo. They're shared
+# infrastructure in a separate Infrastructure repo; ../../Infrastructure
+# below assumes it's checked out as a sibling of Admin. Adjust the path if
+# yours isn't. See that repo's own README.
 dev-infra:
-	docker compose -f deploy/docker-compose.dev.yml up -d
+	docker compose -f ../../Infrastructure/compose.yml -f ../../Infrastructure/compose.dev.yml up -d
 
 dev-infra-down:
-	docker compose -f deploy/docker-compose.dev.yml down
+	docker compose -f ../../Infrastructure/compose.yml -f ../../Infrastructure/compose.dev.yml down
 
 backend:
 	cd backend && go run ./cmd/server