feat(worker): add autosync scheduler, endpoint, locking and UI controls

supabase/migrations/CL6_auto_sync_worker.sql

@@ -0,0 +1,62 @@
+-- CL6: auto-sync worker scheduling fields + per-instance lock helpers
+
+alter table public.plesk_instances
+  add column if not exists auto_sync_enabled boolean not null default false,
+  add column if not exists auto_sync_frequency_minutes integer not null default 60,
+  add column if not exists last_auto_sync_at timestamptz,
+  add column if not exists next_auto_sync_at timestamptz,
+  add column if not exists auto_sync_lock_until timestamptz,
+  add column if not exists auto_sync_lock_token text;
+
+alter table public.plesk_instances
+  drop constraint if exists plesk_instances_auto_sync_frequency_minutes_check;
+
+alter table public.plesk_instances
+  add constraint plesk_instances_auto_sync_frequency_minutes_check
+  check (auto_sync_frequency_minutes between 5 and 10080);
+
+create index if not exists idx_plesk_instances_auto_sync_due
+  on public.plesk_instances (auto_sync_enabled, next_auto_sync_at);
+
+create or replace function public.acquire_instance_auto_sync_lock(
+  p_instance_id uuid,
+  p_lock_token text,
+  p_ttl_seconds integer default 900
+)
+returns boolean
+language plpgsql
+security definer
+set search_path = public
+as $$
+declare
+  v_acquired boolean := false;
+begin
+  update public.plesk_instances
+  set
+    auto_sync_lock_until = now() + make_interval(secs => p_ttl_seconds),
+    auto_sync_lock_token = p_lock_token
+  where id = p_instance_id
+    and auto_sync_enabled = true
+    and (next_auto_sync_at is null or next_auto_sync_at <= now())
+    and (auto_sync_lock_until is null or auto_sync_lock_until < now());
+
+  get diagnostics v_acquired = row_count;
+  return v_acquired;
+end;
+$$;
+
+create or replace function public.release_instance_auto_sync_lock(
+  p_instance_id uuid,
+  p_lock_token text
+)
+returns void
+language sql
+security definer
+set search_path = public
+as $$
+  update public.plesk_instances
+  set auto_sync_lock_until = null,
+      auto_sync_lock_token = null
+  where id = p_instance_id
+    and auto_sync_lock_token = p_lock_token;
+$$;