Split monolithic compose into 5 independent stacks

- docker-compose.infra.yml: core infrastructure (portainer, npm, homepage, wud, etc.)
- docker-compose.media.yml: media stack (arrs, jellyfin, qbittorrent, scrobbling)
- docker-compose.documents.yml: paperless-ngx, onlyoffice, stirling, open-webui
- docker-compose.photo-roms.yml: immich, syncthing, retrom
- docker-compose.utils.yml: gitea, tandoor, speedtest, linkwarden, rustdesk, etc.

Each stack has its own project name (docker-infra, docker-media, etc.) to prevent
orphan warnings. Networks defined in infra.yml, referenced as external by others.
Original preserved as docker-compose.full.yaml.bak.
Updated .gitignore, README, AGENTS.md, and RESTORE.md to reflect new structure.

.gitignore

@@ -2,7 +2,12 @@
 /*
 
 # Only track these files
-!docker-compose.yaml
+!docker-compose.infra.yml
+!docker-compose.media.yml
+!docker-compose.documents.yml
+!docker-compose.photo-roms.yml
+!docker-compose.utils.yml
+!docker-compose.full.yaml.bak
 !.env.example
 !.gitignore
 !README.md