Add token-aware server lifecycle API

[josecelano]

Parent EPIC: #1488 - Overhaul: Tracker Shutdown (roadmap sequence 5, draft SI-2).

Goal

Add and release an additive torrust-server-lib shutdown-wait primitive that resolves from an injected CancellationToken without subscribing to OS signals. Update the tracker to consume that released crate version without migrating existing server consumers.

Scope

• Implement the primitive directly in the existing torrust/torrust-server-lib clone, with deterministic token-cancellation and legacy-compatibility tests.
• Preserve global_shutdown_signal(), shutdown_signal(), shutdown_signal_with_message(), Halted, and existing server start APIs unchanged.
• Open, merge, and publish the focused server-lib change; then update this tracker from torrust-server-lib 0.2.0 to the exact published version.
• Add tracker-side contract coverage confirming the new API is available while existing consumers retain their legacy paths.
• Keep Axum drain-controller ownership and joining out of this task: SI-10 owns the joinable drain helper and SI-11 through SI-17 own individual consumer migrations.

Acceptance Criteria

• A published torrust-server-lib version provides an additive wait primitive that resolves from an injected CancellationToken without subscribing to OS signals.
• Existing global_shutdown_signal(), shutdown_signal(), shutdown_signal_with_message(), and Halted remain source- and behavior-compatible.
• The new primitive does not take or expose Halted; Started startup signaling remains unaffected.
• The tracker consumes the exact published crate version, not an unpublished Git dependency.
• Focused server-lib tests prove token cancellation and legacy behavior; tracker coverage compiles against the new API.
• Future-consumer guidance assigns graceful-stop controller ownership and joining to SI-10 and later component migrations.

Verification

Run server-lib formatting, Clippy, tests, and cargo publish --dry-run; then run tracker linter all, cargo machete, focused server tests, workspace documentation tests, and pre-push checks. Record release and manual cancellation evidence in the issue-local documentation.

Full reviewed specification, including ownership, commit points, manual verification, and risks: docs/issues/open/<issue-number>-1488-si-2-remove-global-shutdown-signal/ISSUE.md after promotion.

Related: #1586, #1588, #2132, #2169, #2221.