Coverage keys are now called integration keys

[MrAlders0n]

Same key, new name. It covers Coverage plus zones, boundaries, scopes, channels and repeaters now, so "Coverage key" read wrong.

• api-keys.md gets a "formerly Coverage keys" note so old links and Discord threads still make sense
• coverage, zones, scopes, channels, admins and faq pages use the new name
• invalid_key message updated to match the server (MeshMapper_Server cf50d1f)

Release notes left alone, they're history.