SI-4: Migrate stats context to contract-first architecture

[josecelano]

Subissue of EPIC #1938 — REST API Contract-First Migration

Problem

The stats context is the most complex in the REST API. It has two endpoints (GET /stats, GET /metrics) aggregating data from 6+ tracker internal repositories across tracker-core, http-core, udp-core, udp-server, swarm-coordination-registry, and rest-api-core. The Stats DTO has ~28 fields. The Axum server injects all dependencies as multi-element state tuples.

Scope

• Define Stats and LabeledStats DTOs (~28 fields) in rest-api-protocol
• Define StatsQueryPort trait in rest-api-application
• Implement StatsApiService use-case
• Implement TrackerStatsAdapter consuming SI-30 traits (BanningStats, UdpCoreStatsRepository, UdpServerStatsRepository)
• Handle Prometheus serialization (keep as Axum transport concern — Option A)
• Rewire Axum handlers to use StatsApiService
• Remove direct internal crate deps from axum-rest-api-server stats wiring

Checklist

• Stats and LabeledStats DTOs defined in rest-api-protocol
• StatsQueryPort trait defined
• StatsApiService use-case implemented
• TrackerStatsAdapter implemented consuming SI-30 traits
• Prometheus serialization handled (Option A)
• Axum handlers dispatch through use-case
• Direct internal crate deps removed from Axum server
• Pre-commit checks pass
• Pre-push checks pass