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>

db/database.py

@@ -85,24 +85,6 @@ class StoredCallFlow(Base):
         return f"<StoredCallFlow {self.id} {self.phone_number}>"
 
 
-class Contact(Base):
-    __tablename__ = "contacts"
-
-    id = Column(String, primary_key=True)
-    name = Column(String, nullable=False)
-    phone_numbers = Column(JSON, nullable=False)  # [{number, label, primary}, ...]
-    category = Column(String)  # personal / business / service
-    routing_preference = Column(String, nullable=True)  # how to handle their calls
-    notes = Column(Text, nullable=True)
-    call_count = Column(Integer, default=0)
-    last_call = Column(DateTime, nullable=True)
-    created_at = Column(DateTime, default=func.now())
-    updated_at = Column(DateTime, default=func.now(), onupdate=func.now())
-
-    def __repr__(self) -> str:
-        return f"<Contact {self.id} {self.name}>"
-
-
 class Device(Base):
     __tablename__ = "devices"