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/gotosocial/README.md

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+# GoToSocial
+
+🐘 ActivityPub Server (Fediverse)
+
+## Deploy in Komodo
+
+**Pfad:** `stacks/gotosocial/`
+
+## Ports
+
+- `8080` - Web UI & API
+
+## Caddy
+
+```caddyfile
+social.example.com {
+    reverse_proxy localhost:8080
+}
+```
+
+## Notes
+
+- ActivityPub-kompatibel (Mastodon, Pixelfed, etc.)
+- SQLite Datenbank (einfach)

stacks/gotosocial/compose.yml

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+services:
+  gotosocial:
+    image: docker.homeserver.net/gotosocial/gotosocial:latest
+    container_name: gotosocial
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:8080:8080"
+    volumes:
+      - gotosocial_data:/gotosocial
+    environment:
+      - GOTOSOCIAL_HOST=example.com
+      - GOTOSOCIAL_DB_TYPE=sqlite
+    networks:
+      - gotosocial-network
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:8080/api/v1/instance"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 30s
+
+volumes:
+  gotosocial_data:
+
+networks:
+  gotosocial-network:
+    driver: bridge

stacks/gotosocial/stack.toml

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+[[stacks]]
+name = "gotosocial"
+description = "ActivityPub Server (Fediverse)"
+compose_path = "compose.yml"