feat: add invoice-configs management functionality

db/queries/invoice_config.sql

@@ -0,0 +1,34 @@
+-- name: GetInvoiceConfigByID :one
+SELECT * FROM invoice_configs
+WHERE id = sqlc.arg(id);
+
+-- name: ListInvoiceConfigs :many
+SELECT * FROM invoice_configs
+ORDER BY created_at DESC;
+
+-- name: CreateInvoiceConfig :one
+INSERT INTO invoice_configs (name, type, description, is_active, metadata, created_at)
+VALUES (
+    sqlc.arg(name),
+    sqlc.arg(type),
+    sqlc.arg(description),
+    sqlc.arg(is_active),
+    sqlc.arg(metadata),
+    sqlc.arg(created_at)
+)
+RETURNING *;
+
+-- name: UpdateInvoiceConfig :one
+UPDATE invoice_configs
+SET name = CASE WHEN sqlc.arg(name) = '' THEN name ELSE sqlc.arg(name) END,
+    type = coalesce(sqlc.arg(type), type),
+    description = coalesce(sqlc.arg(description), description),
+    is_active = coalesce(sqlc.arg(is_active), is_active),
+    metadata = coalesce(sqlc.arg(metadata), metadata),
+    updated_at = sqlc.arg(updated_at)
+WHERE id = sqlc.arg(id)
+RETURNING *;
+
+-- name: DeleteInvoiceConfig :execrows
+DELETE FROM invoice_configs
+WHERE id = sqlc.arg(id);