Stage 4: honest health, explicit error policy, event-bus integrity

Engine mode is now explicit: USE_MOCK_SIP=true is the only way to get
the mock engine; an unconfigured trunk fails startup with guidance
instead of silently degrading. Root-caused why the engine always ran
mock: nested pydantic-settings never read .env (no env_file on the
sub-settings classes) — all 8 now declare it.

/health stops lying: reports engine mode (sippy|mock), a live DB
SELECT 1, trunk registration state with reason, and TTS/STT
availability from their last real request; "healthy" now requires
ready + db + sippy + registered trunk.

Error policy: leaf services (tts/transcription/llm_client) raise and
track availability; call-loop callers catch, publish EventType.ERROR
naming the failed service, and apply an explicit fallback. Persistence
writes get one bounded 3x exponential retry, then an ERROR log — no
more silent data loss.

Event bus: a full subscriber queue drops its oldest event (counted)
instead of silently evicting the subscription; subscribe(replay_last=N)
delivers the advertised history replay, used by /ws/events (25).

Receptionist correctness: a matched TAKE_MESSAGE rule beats the LLM;
voicemail polls for early hangup and stops/transcribes/hangs up in
finally; RecordingSession finally keeps its leg_ids so taps detach.

Dead code removed: models/contact.py + Contact table, dtmf_buffer,
transcribe_stream stub, SMS stub in notification.py.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_concurrency.py

@@ -102,14 +102,9 @@ class TestEngineEventFunnel:
 
     async def test_dtmf_and_trunk_events(self):
         engine = _engine_on_loop()
-        _post_from_thread(engine, "incoming_invite", {
-            "leg_id": "leg_test4", "from_uri": "a", "to_uri": "b", "sdp": None,
-        })
-        await asyncio.sleep(0.05)
         _post_from_thread(engine, "dtmf", {"leg_id": "leg_test4", "digit": "5"})
         _post_from_thread(engine, "trunk_registered", {"registered": True})
         await asyncio.sleep(0.05)
-        assert engine._legs["leg_test4"].dtmf_buffer == ["5"]
         assert engine._trunk_registered is True