SI-33: Define REST API contract-first package architecture for EPIC #1669

[josecelano]

Subissue of EPIC #1669 — Overhaul: Packages

This issue defines and documents a contract-first package architecture for the tracker REST API, so the REST API can evolve toward a reusable standard in future versions while remaining compatible with the current tracker implementation during migration.

This issue defines architecture and migration policy now, but does not implement full API v2 behavior changes yet. It establishes package boundaries and dependency rules that make v2 and standardization feasible.

The full API package refactor is expected to be handled by a dedicated EPIC, separate from EPIC #1669.

Prerequisites

This issue depends on SI-30 (#1924), which delivers the UDP-side trait abstractions (BanningStats, UdpCoreStatsRepository, UdpServerStatsRepository) that the future TrackerStatsAdapter will implement.

Scope

In Scope

• Define target package architecture for REST API contract/application/adapters.
• Define allowed and forbidden dependency edges.
• Define migration phases and compatibility approach for v1/v2.
• PoC branch with one endpoint (torrent detail recommended).
• Consume UDP-side traits from SI-30 (SI-30: Decouple rest-api-core and axum-rest-api-server from concrete UDP server internals #1924).

Out of Scope

• Full API v2 behavior changes (tracked in issue API V2: new API with the correct response codes #144).
• Extracting any package to a standalone repository during EPIC Overhaul: Packages #1669.
• Publishing any REST API package as a stable external contract.
• Changing the HTTP tracker or UDP tracker layers.

Spec

Full specification: docs/issues/open/1930-1669-si-33-rest-api-contract-first-architecture.md