feat(cluster): HA cluster with Raft consensus and TCP replication (v2.5.0)

[caikpigosso]

Summary

Production-grade High Availability cluster implementation for Vectorizer v2.5.0.

• Raft Consensus (openraft 0.10) for metadata operations and leader election
• TCP Replication for streaming vector data from leader to followers
• Write-Redirect Middleware — followers return HTTP 307 redirecting writes to leader
• Dashboard Cluster Page — real-time visualization of nodes, roles, and replication status
• Docker Compose HA — ready-to-use 3-node cluster (docker-compose.ha.yml)
• Kubernetes Support — Helm headless service, DNS discovery, shared JWT auth

Architecture

              ┌─────────────┐
              │ Load Balancer│
              └──────┬──────┘
                     │
        ┌────────────┼────────────┐
        │            │            │
   ┌────▼────┐  ┌───▼───┐  ┌────▼────┐
   │ Leader  │  │Follower│  │Follower │
   │ (write) │  │ (read) │  │ (read)  │
   └────┬────┘  └───▲───┘  └────▲───┘
        └────────────┴───────────┘
            TCP Replication

What's new

Feature	Description
Raft consensus	Leader election, metadata consensus via openraft
TCP replication	Full/partial sync with WAL-backed durability
Write concern	Optional synchronous replication (WAIT command)
HTTP 307 redirect	Followers redirect writes to leader
Epoch conflict resolution	Higher epoch wins shard assignment conflicts
Shard data migration	Actual data transfer during rebalance
Collection quorum	Majority-based collection creation
DNS discovery	K8s headless service auto-discovery
Dashboard	Cluster status page with real-time refresh

Test plan

• 1066 unit/integration tests passing
• Docker HA cluster tested (3 nodes)
• Failover simulation: leader dies → replicas survive → leader recovers
• Write redirect confirmed: replicas return 307
• Replication confirmed: data syncs across all nodes
• cargo fmt, clippy, codespell clean
• No Portuguese text in code or docs

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