openai service and basic tables with migrations

db/migrations/20260102021749_wa_conversation_state.sql

@@ -0,0 +1,23 @@
+-- migrate:up
+create table wa_conversation_state (
+  tenant_id uuid not null references tenants(id) on delete cascade,
+  wa_chat_id text not null,
+
+  state text not null,                 -- IDLE / BUILDING_ORDER / WAITING_PAYMENT
+  last_intent text null,
+  last_order_id bigint null,
+
+  context jsonb not null default '{}'::jsonb,
+
+  state_updated_at timestamptz not null default now(),
+  created_at timestamptz not null default now(),
+  updated_at timestamptz not null default now(),
+
+  primary key (tenant_id, wa_chat_id)
+);
+
+create index idx_state_tenant_updated
+  on wa_conversation_state (tenant_id, updated_at desc);
+
+-- migrate:down
+drop table if exists wa_conversation_state;