feat: Add final stacks (Batch 3)

Added stacks:
- plex (Media server)
- pterodactyl-panel (Game server management)
- gotosocial (ActivityPub/Fediverse)
- backrest (Backup UI)

All stacks include:
- compose.yml with proper configuration
- README.md with deployment info
- stack.toml for Komodo Resource Sync

TOTAL: 21 stacks ready for Komodo deployment!

stacks/pterodactyl-panel/README.md

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+# Pterodactyl Panel
+
+🎮 Game Server Management
+
+## Deploy in Komodo
+
+**Pfad:** `stacks/pterodactyl-panel/`
+
+## Ports
+
+- `8080` - HTTP
+- `8443` - HTTPS
+
+## Caddy
+
+```caddyfile
+panel.example.com {
+    reverse_proxy localhost:8080
+}
+```
+
+## Notes
+
+- Komplexes Setup mit MariaDB + Redis
+- Benötigt Pterodactyl Wings auf Game-Servern

stacks/pterodactyl-panel/compose.yml

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+services:
+  pterodactyl-panel:
+    image: ghcr.io/pterodactyl/panel:latest
+    container_name: pterodactyl-panel
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:8080:80"
+      - "127.0.0.1:8443:443"
+    volumes:
+      - pterodactyl_panel_var:/var/www/pterodactyl/var
+      - pterodactyl_panel_nginx:/etc/ssl/pterodactyl
+    environment:
+      - TRUSTED_PROXIES=*
+    networks:
+      - pterodactyl-network
+    depends_on:
+      - pterodactyl-db
+      - pterodactyl-cache
+
+  pterodactyl-db:
+    image: mariadb:10.5
+    container_name: pterodactyl-db
+    restart: unless-stopped
+    volumes:
+      - pterodactyl_db:/var/lib/mysql
+    environment:
+      - MYSQL_DATABASE=panel
+      - MYSQL_USER=pterodactyl
+      - MYSQL_PASSWORD=pterodactyl
+      - MYSQL_ROOT_PASSWORD=rootpassword
+    networks:
+      - pterodactyl-network
+
+  pterodactyl-cache:
+    image: redis:alpine
+    container_name: pterodactyl-cache
+    restart: unless-stopped
+    volumes:
+      - pterodactyl_cache:/data
+    networks:
+      - pterodactyl-network
+
+volumes:
+  pterodactyl_panel_var:
+  pterodactyl_panel_nginx:
+  pterodactyl_db:
+  pterodactyl_cache:
+
+networks:
+  pterodactyl-network:
+    driver: bridge

stacks/pterodactyl-panel/stack.toml

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+[[stacks]]
+name = "pterodactyl-panel"
+description = "Game Server Management"
+compose_path = "compose.yml"