feat(sprint6b-D): soft-insertion counter + message meta_json

- thread_state.soft_insertion_count: растёт при боковом ответе (soft_insertion=true
  в STATE_JSON без смены шага/слотов), сбрасывается при продвижении или handoff
- При soft_insertion_count >= 3 в системный промпт ветки добавляется SOFT_INSERTION_NUDGE
  — явная инструкция вернуть пациента к вопросу текущего шага
- state_machine.parse_branch_response читает флаг soft_insertion из STATE_JSON
- Новая колонка message.meta_json: {router_intent_code, served_intent_code, step_code, events}
  — хранит снимок маршрутизации каждой реплики ассистента
- «Песочница»: бейджи событий (sticky / soft_insertion / hard_handoff / resumed /
  routing_loop / validation_blocked) над каждым ответом ассистента

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/thread_state_service.py

@@ -39,6 +39,7 @@ async def load_snapshot(session: AsyncSession, thread_id: int) -> dict:
             "current_step_code": None,
             "slots": {},
             "handoff_count": 0,
+            "soft_insertion_count": 0,
             "suspended_intent": None,
             "resumable_step_code": None,
             "resumable_slots": {},
@@ -57,6 +58,7 @@ async def load_snapshot(session: AsyncSession, thread_id: int) -> dict:
         "current_step_code": state.current_step_code,
         "slots": _parse_slots(state.slots_json),
         "handoff_count": state.handoff_count,
+        "soft_insertion_count": state.soft_insertion_count,
         "suspended_intent": state.suspended_intent,
         "resumable_step_code": state.resumable_step_code,
         "resumable_slots": resumable_slots,
@@ -72,6 +74,7 @@ async def upsert(
     slots: dict,
     step_code: str | None = None,
     handoff_count: int = 0,
+    soft_insertion_count: int = 0,
     suspended_intent: str | None = None,
     resumable_step_code: str | None = None,
     resumable_slots: dict | None = None,
@@ -93,6 +96,7 @@ async def upsert(
             current_step_code=step_code,
             slots_json=slots_raw,
             handoff_count=handoff_count,
+            soft_insertion_count=soft_insertion_count,
             suspended_intent=suspended_intent,
             resumable_step_code=resumable_step_code,
             resumable_slots_json=resumable_raw,
@@ -105,6 +109,7 @@ async def upsert(
         state.current_step_code = step_code
         state.slots_json = slots_raw
         state.handoff_count = handoff_count
+        state.soft_insertion_count = soft_insertion_count
         state.suspended_intent = suspended_intent
         state.resumable_step_code = resumable_step_code
         state.resumable_slots_json = resumable_raw