docs: sync release metadata for v2.15.1

[szibis]

Summary

• sync release metadata files for v2.15.1 after the automated release
• CHANGELOG materialization, Helm chart version/appVersion, observability service.version, README release badges

[github-actions]

PR Quality Report

Compared against base branch main.

Coverage and tests

Signal	Base	PR	Delta
Test count	9739	9739	0
Coverage	87.9%	87.8%	-0.1% (regressed)

Compatibility

Track	Base	PR	Delta
Loki API	100.0%	11/11 (100.0%)	0.0% (stable)
Logs Drilldown	100.0%	17/17 (100.0%)	0.0% (stable)
VictoriaLogs	100.0%	11/11 (100.0%)	0.0% (stable)

Performance smoke

Performance smoke was skipped for this PR because no perf-sensitive paths changed.

State

• Coverage, compatibility, and sampled performance are reported here from the same PR workflow.
• This is a delta report, not a release gate by itself. Required checks still decide merge safety.
• Performance smoke was intentionally skipped because no perf-sensitive paths changed in this PR.
• Delta states use the same noise guards as the quality gate (percent + absolute + low-baseline checks), so report labels match merge-gate behavior.