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>

db/database.py

@@ -4,6 +4,8 @@ Database connection and session management.
 PostgreSQL via asyncpg + SQLAlchemy async.
 """
 
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
 from datetime import datetime
 
 from sqlalchemy import (
@@ -204,8 +206,13 @@ def get_session_factory() -> async_sessionmaker[AsyncSession]:
     return _session_factory
 
 
-async def get_db() -> AsyncSession:
-    """Dependency: yield an async database session."""
+@asynccontextmanager
+async def session_scope() -> AsyncIterator[AsyncSession]:
+    """A commit-on-success session — the one session-lifecycle convention.
+
+    REST handlers get it via the get_db dependency; services and MCP
+    tools use it directly.
+    """
     factory = get_session_factory()
     async with factory() as session:
         try:
@@ -216,6 +223,12 @@ async def get_db() -> AsyncSession:
             raise
 
 
+async def get_db() -> AsyncIterator[AsyncSession]:
+    """FastAPI dependency: yield an async database session."""
+    async with session_scope() as session:
+        yield session
+
+
 async def init_db():
     """Create all tables. For development; use Alembic migrations in production."""
     engine = get_engine()