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>

core/sippy_engine.py

@@ -50,7 +50,6 @@ class SipCallLeg:
         self.state = "init"  # init, trying, ringing, connected, terminated
         self.media_port: Optional[int] = None  # PJSUA2 conf bridge port
         self.pending_sdp: Optional[str] = None  # inbound INVITE SDP, until answered
-        self.dtmf_buffer: list[str] = []
 
     def __repr__(self):
         return f"<SipCallLeg {self.leg_id} {self.direction} {self.state} → {self.remote_uri}>"
@@ -297,9 +296,8 @@ class SippyEngine(SIPEngine):
             ]
 
         elif kind == "dtmf":
-            leg = self._legs.get(data["leg_id"])
-            if leg:
-                leg.dtmf_buffer.append(data["digit"])
+            # Received DTMF has no consumer yet; log until one exists
+            logger.info(f"  DTMF '{data['digit']}' received on {data['leg_id']}")
 
         elif kind == "trunk_registered":
             self._trunk_registered = data["registered"]