fix: Release the microphone to the browser when a call ends (#2685)

The microphone is connected when a call needs it and released the moment
that
call ends, so the browser stops reporting it as in use at hangup rather
than
`disconnect_seconds` later.

Existing configurations are migrated automatically by the visual editor.

 - Closes: #2681 

BREAKING CHANGE: `live.microphone.disconnect_seconds` is removed. The
microphone is released when a call ends, so there is no idle countdown
to
configure. Use `live.microphone.always_connected` to hold it open
instead.

BREAKING CHANGE: The `microphone_connect` and `microphone_disconnect`
actions
are removed. The card owns the microphone lifecycle; `microphone_mute`
and
`microphone_unmute` remain.

BREAKING CHANGE: `call` is removed from `live.microphone.auto_mute`,
whose
default is now `[]`. The microphone is muted when a call ends regardless
of
this option.

BREAKING CHANGE: `microphone_unmute` has no effect outside a call.
Nothing
carries the audio at any other time, so the request is ignored rather
than
opening the microphone.

docs/uml/call-sequence.puml

@@ -62,9 +62,9 @@ note over User, Card
 end note
 
 Card --> User : Inbound audio mutes (<color:#43a047>**""live.auto_mute""**</color>)
-Card -> Mic : Microphone mutes (<color:#43a047>**""live.microphone.auto_mute""**</color>)
+Card -> Mic : Microphone mutes and disconnects
 note over Mic
-  Disconnects after <color:#43a047>**""live.microphone.disconnect_seconds""**</color>
-  unless <color:#43a047>**""live.microphone.always_connected""**</color>
+  The browser stops reporting the microphone as in use, unless
+  <color:#43a047>**""live.microphone.always_connected""**</color> holds it open
 end note
 @enduml