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>

services/llm_client.py

@@ -327,15 +327,14 @@ class LLMClient:
         except httpx.HTTPStatusError as e:
             self._total_errors += 1
             logger.error(f"LLM API error: {e.response.status_code} {e.response.text[:200]}")
-            return ""
+            raise
         except httpx.TimeoutException:
             self._total_errors += 1
             logger.error(f"LLM API timeout after {self.timeout}s")
-            return ""
-        except Exception as e:
+            raise
+        except Exception:
             self._total_errors += 1
-            logger.error(f"LLM client error: {e}")
-            return ""
+            raise
 
     @staticmethod
     def _parse_json_response(text: str) -> dict[str, Any]: