Define canonical runtime service identity

[josecelano]

Goal

Define tracker-owned canonical service-role and configuration-instance identity types that can be used consistently by bootstrap, runtime registration, and event-metrics consumers.

Background

The earlier #2036 plan combined two deliveries:

1. defining canonical tracker identity types; and
2. migrating torrust-server-lib::Registar and tracker registrations to carry them.

The registration migration cannot be completed until #2035 propagates identity through actual bootstrap. The migration is therefore split into #2041. This issue owns only the independently mergeable canonical type foundation.

Scope

• Define tracker-owned canonical service-role values without coupling generic server/network libraries to tracker variants.
• Define canonical configuration-instance identity with clear scope and equality semantics.
• Document and unit-test those types.
• Verify Fix duplicate port-zero tracker instance bootstrap #2035, Migrate runtime service registry metadata #2041, and Normalize per-instance event metrics policy #2039 can consume them without competing identity models.

Out of scope

• Duplicate port-zero bootstrap storage: Fix duplicate port-zero tracker instance bootstrap #2035.
• torrust-server-lib registration/query/readiness API and release: Migrate runtime service registry metadata #2041.
• Event-metrics listener filtering: Normalize per-instance event metrics policy #2039.

Implementation outline

1. Define tracker-owned service role type.
2. Define canonical configuration-instance identity type.
3. Verify intended consumer boundaries.
4. Run focused tests and linter all.

Acceptance criteria

• Canonical service-role values remain tracker-owned.
• Configuration-instance identity is typed, documented, and independent of configured socket addresses.
• Fix duplicate port-zero tracker instance bootstrap #2035, Migrate runtime service registry metadata #2041, and Normalize per-instance event metrics policy #2039 can use the same type without conversion to competing identity types.
• Focused tests and linter all pass.

Related to #2035, #2039, and #2041.