fix: handle existing index in deduplicate_discovery_checks migration (#179)

- Drop index if exists before creating to handle manual creation
- Prevents duplicate_table error when index was created manually

Reviewed-on: graham/towerops-web#179

priv/repo/migrations/20260325215632_deduplicate_discovery_checks.exs

@@ -33,6 +33,9 @@ defmodule Towerops.Repo.Migrations.DeduplicateDiscoveryChecks do
 
     # Prevent future duplicates for auto-discovered checks.
     # Use concurrently: true to avoid locking table during index creation.
+    # Drop first if it exists (may have been created manually)
+    execute("DROP INDEX IF EXISTS checks_device_type_source_unique_index")
+
     create(
       unique_index(:checks, [:device_id, :check_type, :source_id],
         where: "source_type = 'auto_discovery' AND source_id IS NOT NULL",