The policy builder only offered "Matches everything" (the `true` scope node),
which also targets servers. Authoring an estate-wide policy almost always means
"every PC an operator sits at", and a reboot or script push against a server is
a materially different event.
Adds an `allpcs` scope node that matches every endpoint except those whose
computer type is server-designated. Server designation reads the computer type's
ICON slug, which is nominally a UI hint and is now load-bearing; it was chosen
over matching the type NAME (a site can rename "Server") and over a new isserver
column (needs a migration). It is isolated behind SERVER_ICON so promoting it to
a real flag later is a one-place change.
The `true` node is retained and still matches servers, so existing policies keep
their exact meaning rather than silently re-scoping. It stays visible in the
dropdown only when editing a policy that already uses it; new policies default
to `allpcs`.
Covered by tests/test_plugins/test_computers_policy_allpcs.py, verified against
a planted canary so the server-exclusion assertions are known to bite.
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