Rebalance swarm: promote all nodes to managers and remove hostname constraints

- Promoted p1, p2, p3 from worker to manager nodes for 4-node quorum
- Removed unnecessary hostname constraints from service configs
- Only traefik and portainer remain pinned to p0
- Services now auto-balance across all nodes via GlusterFS shared storage
- Updated README with cluster overview and distribution strategy

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

stacks/apps/uptime/stack.yml

@@ -14,9 +14,10 @@ services:
       retries: 3
       start_period: 60s
     deploy:
-      placement:
-        constraints: [node.hostname == p0]
       replicas: 1
+      placement:
+        preferences:
+        - spread: node.hostname
       restart_policy:
         condition: on-failure
         delay: 10s