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>

api/deps.py

@@ -18,6 +18,14 @@ def get_gateway(request: Request) -> AIPSTNGateway:
     return gateway
 
 
+def get_routing_service(request: Request):
+    """Get the routing service from app state."""
+    routing = getattr(request.app.state, "routing_service", None)
+    if routing is None:
+        raise HTTPException(status_code=503, detail="Routing service not ready")
+    return routing
+
+
 def require_token(authorization: str | None = Header(default=None)) -> None:
     """
     Enforce the static bearer token (API_TOKEN) on REST routes.