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/calls.py

@@ -172,10 +172,9 @@ async def send_dtmf(
     if not call:
         raise HTTPException(status_code=404, detail=f"Call {call_id} not found")
 
-    # Find the PSTN leg for this call
-    for leg_id, cid in gateway.call_manager._call_legs.items():
-        if cid == call_id:
-            await gateway.sip_engine.send_dtmf(leg_id, digits)
-            return {"status": "sent", "digits": digits}
+    legs = gateway.call_manager.legs_for_call(call_id)
+    if not legs:
+        raise HTTPException(status_code=409, detail="No active SIP leg found for this call")
 
-    raise HTTPException(status_code=500, detail="No active SIP leg found for this call")
+    await gateway.sip_engine.send_dtmf(legs[0], digits)
+    return {"status": "sent", "digits": digits}