db: connection helper and invoices migration

DB.connect! opens a single Sequel connection from DATABASE_URL; DB.migrate!
runs Sequel migrations from db/migrations/.

The first migration creates the invoices table: uuid PK, unique invoice
number, client + currency fields, JSONB for line items, GEL conversion rate
captured at issue time, paid_at for the status toggle, pdf_key for the
MinIO object.

lib/db.rb

@@ -0,0 +1,22 @@
+require 'sequel'
+
+module DB
+  module_function
+
+  def connect!
+    @connection ||= Sequel.connect(
+      ENV.fetch('DATABASE_URL'),
+      max_connections: 5,
+      logger:          ($env == 'development' ? $logger : nil)
+    )
+  end
+
+  def connection
+    @connection or raise 'DB not connected — call DB.connect! first'
+  end
+
+  def migrate!
+    Sequel.extension :migration
+    Sequel::Migrator.run(connection, File.join($root, 'db', 'migrations'))
+  end
+end