refactor: composition root in lifespan, break core↔services cycle, shared data layer

The gateway was the composition root, device registry, inbound-call
policy, and call-operations service in one class, with core↔services
circular imports papered over by function-local imports, wiring done
by assigning private attributes, and MCP tools duplicating REST query
logic against their own sessions.

Composition:
- main.py's lifespan now builds every service and wires them by
  constructor/registration. gateway.from_config() is gone; core/ no
  longer imports services/ anywhere — the cycle is dead.
- Inbound-call policy moved to ReceptionistService.on_inbound_call
  (routing evaluation, reject/answer, screening dispatch); wired as
  the engine's on_incoming_call by the lifespan. Receptionist deps
  (tts/transcription/recording/routing) are constructor-injected —
  no more gateway._tts reach-through or importing hold_slayer's
  private _get_llm (now services.llm_client.get_llm, shared).
- Hold-slayer launch goes through a mode-handler registry
  (register_mode_handler); the gateway no longer knows the service's
  type. CallManager takes on_call_ended in its constructor.
- build_sip_engine() is a pure function taking explicit callbacks.
- api/routing.py uses the routing service from app.state via a
  proper dependency instead of gateway._routing.

Shared data layer:
- db.session_scope() is the one session convention (get_db wraps it).
- services/call_persistence.py gains the query/write functions and
  the single StoredCallFlow→CallFlow mapper; api/call_flows.py,
  api/call_history.py, and the six DB-touching MCP tools are thin
  wrappers over them — the two surfaces can't drift.
- legs_for_call() replaces the three private _call_legs scans
  (gateway transfer/hangup, REST dtmf, MCP dtmf).

7 new tests (mode-handler launch, on_call_ended hook, receptionist
inbound answer/reject, call-flow CRUD round-trip and history routes
against real SQLite through the shared layer). aiosqlite added to dev
deps for that.

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

services/llm_client.py

@@ -389,3 +389,31 @@ class LLMClient:
             "model": self.model,
             "base_url": self.base_url,
         }
+
+
+# ================================================================
+# Shared lazy client
+# ================================================================
+
+_shared_client: Optional["LLMClient"] = None
+_shared_failed = False
+
+
+def get_llm() -> Optional["LLMClient"]:
+    """Lazily build the shared LLMClient from settings (None if unavailable)."""
+    global _shared_client, _shared_failed
+    if _shared_client is None and not _shared_failed:
+        try:
+            from config import get_settings
+
+            settings = get_settings()
+            _shared_client = LLMClient(
+                base_url=settings.llm.base_url,
+                model=settings.llm.model,
+                api_key=settings.llm.api_key.get_secret_value(),
+                timeout=settings.llm.timeout,
+            )
+        except Exception as e:
+            logger.debug(f"LLM client not available: {e}")
+            _shared_failed = True  # don't retry
+    return _shared_client