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.

backend/.env.example

@@ -2,14 +2,21 @@ APP_ENV=development
 PORT=8090
 CORS_ALLOWED_ORIGINS=http://localhost:5173
 
-DATABASE_URL=postgres://dockermanager:dockermanager@localhost:5432/dockermanager?sslmode=disable
+# Postgres, Zitadel and SeaweedFS come from the shared ../../Infrastructure
+# stack (`make dev-infra`), not from a container in this repo. Credentials
+# must match Infrastructure's own .env.
+DATABASE_URL=postgres://postgres:postgres@host.docker.internal:5432/dockermanager_db?sslmode=disable
 
-ZITADEL_ISSUER=http://localhost:8081
+# Not wired into the backend yet (see README). These point at Infrastructure's
+# Zitadel for when OIDC login is implemented.
+ZITADEL_ISSUER=http://host.docker.internal:8080
 ZITADEL_CLIENT_ID=
 ZITADEL_CLIENT_SECRET=
 SESSION_SECRET=change-me-in-production
 
-S3_ENDPOINT=http://localhost:8333
+# Not wired into the backend yet. Points at Infrastructure's SeaweedFS for
+# when file storage is implemented.
+S3_ENDPOINT=http://host.docker.internal:8333
 S3_BUCKET=dockermanager
 S3_ACCESS_KEY=
 S3_SECRET_KEY=