docs(changelog): cut v1.5.6 bundled release
Closes the v1.5.5 regression chain (issue #40, both the apparent
ffmpeg exit status 251 symptom and the follow-up Sonarr 401),
folds in the validated audit findings from the v1.4.0..v1.5.5
sweep (~23% of agent-flagged items were false positives, only the
verified ones ship), and adds the first diagnostic endpoint so this
class of bug becomes catchable in CI from now on.