fix: add missing foreign key indexes for performance and referential integrity (#167)

Missing indexes detected on 7 foreign key columns:
- on_call_overrides.user_id
- on_call_layer_members.user_id
- site_outages.site_id
- escalation_targets.user_id
- notification_digests.organization_id
- on_call_notifications.user_id
- alerts.site_outage_id (already existed, skipped)

Without indexes, PostgreSQL must perform sequential scans for:
- Referential integrity checks on DELETE/UPDATE of parent rows
- JOIN operations on these columns
- Can cause lock contention and slow queries

Indexes created with CONCURRENTLY option to avoid blocking production
tables during deployment.

Migration uses @disable_ddl_transaction and @disable_migration_lock
to ensure indexes are built without holding locks.

Reviewed-on: graham/towerops-web#167

priv/repo/migrations/20260325180211_add_missing_fk_indexes.exs

@@ -0,0 +1,44 @@
+defmodule Towerops.Repo.Migrations.AddMissingFkIndexes do
+  use Ecto.Migration
+
+  @disable_ddl_transaction true
+  @disable_migration_lock true
+
+  def up do
+    # Create indexes concurrently to avoid locking tables in production
+    # These foreign keys were missing indexes, causing slow DELETEs/UPDATEs
+    # on parent tables and slow JOINs
+
+    # on_call_overrides.user_id -> users.id
+    create_if_not_exists index(:on_call_overrides, [:user_id], concurrently: true)
+
+    # on_call_layer_members.user_id -> users.id
+    create_if_not_exists index(:on_call_layer_members, [:user_id], concurrently: true)
+
+    # site_outages.site_id -> sites.id
+    create_if_not_exists index(:site_outages, [:site_id], concurrently: true)
+
+    # escalation_targets.user_id -> users.id
+    create_if_not_exists index(:escalation_targets, [:user_id], concurrently: true)
+
+    # notification_digests.organization_id -> organizations.id
+    create_if_not_exists index(:notification_digests, [:organization_id], concurrently: true)
+
+    # on_call_notifications.user_id -> users.id
+    create_if_not_exists index(:on_call_notifications, [:user_id], concurrently: true)
+
+    # alerts.site_outage_id -> site_outages.id
+    create_if_not_exists index(:alerts, [:site_outage_id], concurrently: true)
+  end
+
+  def down do
+    # Drop indexes in reverse order
+    drop_if_exists index(:alerts, [:site_outage_id], concurrently: true)
+    drop_if_exists index(:on_call_notifications, [:user_id], concurrently: true)
+    drop_if_exists index(:notification_digests, [:organization_id], concurrently: true)
+    drop_if_exists index(:escalation_targets, [:user_id], concurrently: true)
+    drop_if_exists index(:site_outages, [:site_id], concurrently: true)
+    drop_if_exists index(:on_call_layer_members, [:user_id], concurrently: true)
+    drop_if_exists index(:on_call_overrides, [:user_id], concurrently: true)
+  end
+end