Add Gitea backup/restore scripts, parameterize configs

- Add gitea/backup.sh and gitea/restore.sh
- Move hardcoded values in gitea/docker-compose.yml to gitea/.env
- Move immich .env from root to immich-app/, update env_file path
- Remove root docker-compose.yml (was only an include alias)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gitea/.env

@@ -0,0 +1,7 @@
+GITEA_DATA=/mnt/misc/gitea
+SSH_KEY_PATH=/home/git/.ssh
+DB_DATA_LOCATION=/mnt/ssd/dbs/gitea/postgres
+DB_USER=gitea
+DB_PASSWORD=gitea
+DB_NAME=gitea
+BACKUP_DIR=/mnt/backups/gitea