openai service and basic tables with migrations

db/migrations/20260102021907_conversation_runs.sql

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+-- migrate:up
+create table conversation_runs (
+  id uuid primary key default gen_random_uuid(),
+  tenant_id uuid not null references tenants(id) on delete cascade,
+  wa_chat_id text not null,
+  message_id text not null,
+
+  ts timestamptz not null default now(),
+  prev_state text null,
+  user_text text null,
+
+  llm_output jsonb null,
+  tools jsonb not null default '[]'::jsonb,
+  invariants jsonb not null default '{}'::jsonb,
+
+  final_reply text null,
+  order_id bigint null,
+  payment_link text null,
+
+  status text not null default 'ok', -- ok | warn | error
+  error_code text null,
+  error_detail text null,
+  latency_ms int null,
+
+  unique (tenant_id, message_id)
+);
+
+create index idx_runs_tenant_chat_ts
+  on conversation_runs (tenant_id, wa_chat_id, ts desc);
+
+-- migrate:down
+drop table if exists conversation_runs;