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/backrest/compose.yml

@@ -0,0 +1,25 @@
+services:
+  backrest:
+    image: ghcr.io/gabm/backrest:latest
+    container_name: backrest
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:8080:8080"
+    volumes:
+      - backrest_config:/config
+      - /backups:/backups
+    networks:
+      - backrest-network
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:8080/api/health"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 10s
+
+volumes:
+  backrest_config:
+
+networks:
+  backrest-network:
+    driver: bridge