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>

models/contact.py

@@ -1,60 +0,0 @@
-"""
-Contact models — People and organizations you call.
-"""
-
-from datetime import datetime
-from typing import Optional
-
-from pydantic import BaseModel, Field
-
-
-class PhoneNumber(BaseModel):
-    """A phone number associated with a contact."""
-
-    number: str  # E.164 format
-    label: str = "main"  # main, mobile, work, home, fax, etc.
-    primary: bool = False
-
-
-class ContactBase(BaseModel):
-    """Shared contact fields."""
-
-    name: str
-    phone_numbers: list[PhoneNumber]
-    category: Optional[str] = None  # personal / business / service
-    routing_preference: Optional[str] = None  # how to handle their calls
-    notes: Optional[str] = None
-
-
-class Contact(ContactBase):
-    """Full contact model."""
-
-    id: str
-    call_count: int = 0
-    last_call: Optional[datetime] = None
-    created_at: Optional[datetime] = None
-    updated_at: Optional[datetime] = None
-
-    @property
-    def primary_number(self) -> Optional[str]:
-        """Get the primary phone number."""
-        for pn in self.phone_numbers:
-            if pn.primary:
-                return pn.number
-        return self.phone_numbers[0].number if self.phone_numbers else None
-
-
-class ContactCreate(ContactBase):
-    """Request model for creating a contact."""
-
-    pass
-
-
-class ContactUpdate(BaseModel):
-    """Request model for updating a contact."""
-
-    name: Optional[str] = None
-    phone_numbers: Optional[list[PhoneNumber]] = None
-    category: Optional[str] = None
-    routing_preference: Optional[str] = None
-    notes: Optional[str] = None