feat(db): add instance health monitoring fields

supabase/migrations/CL38_instance_health_monitoring.sql

@@ -0,0 +1,78 @@
+-- CL8: instance health monitoring fields + lock helpers
+
+alter table public.plesk_instances
+  add column if not exists health_enabled boolean not null default true,
+  add column if not exists health_status text not null default 'unknown',
+  add column if not exists health_last_checked_at timestamptz,
+  add column if not exists health_last_ok_at timestamptz,
+  add column if not exists health_last_error text,
+  add column if not exists health_last_latency_ms integer,
+  add column if not exists health_check_frequency_minutes integer not null default 30,
+  add column if not exists health_next_check_at timestamptz,
+  add column if not exists health_lock_until timestamptz,
+  add column if not exists health_lock_token text;
+
+alter table public.plesk_instances
+  drop constraint if exists plesk_instances_health_status_check;
+
+alter table public.plesk_instances
+  add constraint plesk_instances_health_status_check
+  check (health_status in ('ok', 'degraded', 'down', 'unknown'));
+
+alter table public.plesk_instances
+  drop constraint if exists plesk_instances_health_check_frequency_minutes_check;
+
+alter table public.plesk_instances
+  add constraint plesk_instances_health_check_frequency_minutes_check
+  check (health_check_frequency_minutes between 5 and 10080);
+
+create index if not exists idx_plesk_instances_health_due
+  on public.plesk_instances (health_enabled, health_next_check_at);
+
+create or replace function public.acquire_instance_health_lock(
+  p_instance_id uuid,
+  p_lock_token text,
+  p_ttl_seconds integer default 300,
+  p_force boolean default false
+)
+returns boolean
+language plpgsql
+security definer
+set search_path = public
+as $$
+declare
+  v_acquired boolean := false;
+begin
+  update public.plesk_instances
+  set
+    health_lock_until = now() + make_interval(secs => p_ttl_seconds),
+    health_lock_token = p_lock_token
+  where id = p_instance_id
+    and health_enabled = true
+    and (
+      p_force = true
+      or health_next_check_at is null
+      or health_next_check_at <= now()
+    )
+    and (health_lock_until is null or health_lock_until < now());
+
+  get diagnostics v_acquired = row_count;
+  return v_acquired;
+end;
+$$;
+
+create or replace function public.release_instance_health_lock(
+  p_instance_id uuid,
+  p_lock_token text
+)
+returns void
+language sql
+security definer
+set search_path = public
+as $$
+  update public.plesk_instances
+  set health_lock_until = null,
+      health_lock_token = null
+  where id = p_instance_id
+    and health_lock_token = p_lock_token;
+$$;