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/plex/README.md

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+# Plex
+
+🎬 Media Server (Commercial)
+
+## Deploy in Komodo
+
+**Pfad:** `stacks/plex/`
+
+## Ports
+
+- `32400` - Web UI & API
+
+## Caddy
+
+```caddyfile
+plex.example.com {
+    reverse_proxy localhost:32400
+}
+```
+
+## Notes
+
+- PLEX_CLAIM Token von plex.tv nötig
+- Hardware Transcoding: `/dev/dri` mounten

stacks/plex/compose.yml

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+services:
+  plex:
+    image: plexinc/pms-docker:latest
+    container_name: plex
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:32400:32400"
+    volumes:
+      - plex_config:/config
+      - plex_transcode:/transcode
+      - /media:/media:ro
+    environment:
+      - PLEX_CLAIM=claim-token-from-plex.tv
+    devices:
+      - /dev/dri:/dev/dri
+    networks:
+      - plex-network
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:32400/identity"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 60s
+
+volumes:
+  plex_config:
+  plex_transcode:
+
+networks:
+  plex-network:
+    driver: bridge

stacks/plex/stack.toml

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+[[stacks]]
+name = "plex"
+description = "Media Server (Commercial)"
+compose_path = "compose.yml"