feat: complete remaining Go backend modules

database/migrations/0002_generation_lifecycle_fencing.sql

@@ -0,0 +1,69 @@
+-- Recoverable generation finalization and charge-before-dispatch fencing.
+-- Additive so the previous release remains rollback-compatible.
+
+alter table public.generation_jobs
+  add column if not exists dispatch_ready_at timestamptz;
+
+alter table public.generation_jobs
+  add column if not exists finalized_at timestamptz;
+
+alter table public.generation_jobs
+  add column if not exists provider_dispatch_started_at timestamptz;
+
+-- Existing rows were created before dispatch fencing and are therefore ready.
+update public.generation_jobs
+set dispatch_ready_at = coalesce(dispatch_ready_at, created_at, now())
+where dispatch_ready_at is null;
+
+-- Rows already terminal before this migration have completed their legacy
+-- finalization path and must not be replayed into billing/webhook side effects.
+update public.generation_jobs
+set finalized_at = coalesce(finalized_at, completed_at, updated_at, now())
+where finalized_at is null
+  and status in ('succeeded', 'failed', 'expired', 'cancelled');
+
+create index if not exists generation_jobs_finalize_claim_idx
+  on public.generation_jobs(finalized_at, scheduled_at, locked_at, priority desc);
+
+create or replace function public.claim_generation_jobs(
+  p_worker_id text,
+  p_limit integer default 1,
+  p_lock_timeout_seconds integer default 300
+)
+returns setof public.generation_jobs
+language plpgsql
+set search_path = public, pg_temp
+as $$
+declare
+  v_now timestamptz := now();
+begin
+  return query
+  with candidates as (
+    select id
+    from public.generation_jobs
+    where dispatch_ready_at is not null
+      and finalized_at is null
+      and status in ('queued', 'running', 'succeeded', 'failed', 'expired', 'cancelled')
+      and coalesce(scheduled_at, created_at) <= v_now
+      and (
+        locked_at is null
+        or locked_at < v_now - make_interval(secs => p_lock_timeout_seconds)
+      )
+    order by coalesce(priority, 0) desc, coalesce(scheduled_at, created_at) asc, created_at asc
+    limit greatest(1, least(coalesce(p_limit, 1), 20))
+    for update skip locked
+  ),
+  updated as (
+    update public.generation_jobs
+    set locked_at = v_now,
+        locked_by = p_worker_id,
+        started_at = coalesce(public.generation_jobs.started_at, v_now),
+        updated_at = v_now
+    where id in (select id from candidates)
+    returning public.generation_jobs.*
+  )
+  select * from updated;
+end;
+$$;
+
+revoke all on function public.claim_generation_jobs(text, integer, integer) from public;