Parent Call coordinator

Sits between the classrooms, Companion and ProPresenter, and owns the
truth about a parent call: validation, ids, the queue, expiry,
duplicates, mode, approval, delivery, and every status a classroom is
told. Companion is an operator surface and holds no state.

Built after querying the church's own ProPresenter rather than working
from the assumed contract, which turned out to be wrong in two ways: the
message is called Parent Caller, not Parent Call, and its tokens are
Name and Room rather than child and room.

The finding that shaped the design is recorded in
docs/propresenter-api.md. A trigger carrying a token name the message
does not have still returns 204, and still puts the message on screen
showing the previous call's values. So a 204 is not evidence on its own:
the adapter reads the live message and refuses to trigger unless the
configured tokens exist in it. Without that, a typo in .env would call
the wrong child's parents and report success.

Nothing claims more than it knows. A successful API call is reported as
sent to sanctuary, never as displayed.

The state machine is pure and synchronous, with time passed in and
effects returned, so the awkward cases are testable properly: two rooms
calling at once, an approval for the wrong id, an approval arriving
after the request went stale, and a clear timer firing after the next
call has gone up. The queue is in memory and dies with the process,
because a parent call surfacing ten minutes late would be worse than one
that was lost.

81 tests, then the whole thing end to end on Control: a request through
the real broker and the real coordinator to the real ProPresenter, with
the correct child and room on the sanctuary screen, the coordinator's
own clear timer taking it down again, mode off sending nothing, and a
duplicate id not sending twice.

tests/conftest.py

@@ -0,0 +1,4 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))