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.

db/migrations/001_invoices.rb

@@ -0,0 +1,23 @@
+Sequel.migration do
+  change do
+    create_table(:invoices) do
+      column :uuid,           :uuid,    primary_key: true
+      String  :number,        null: false, unique: true
+      String  :client_name,   null: false
+      String  :client_email,  null: false
+      String  :client_address, text: true
+      String  :currency,      null: false, size: 3
+      BigDecimal :gel_rate,   size: [12, 4], null: false
+      BigDecimal :subtotal,   size: [14, 2], null: false
+      column :items,          :jsonb, null: false
+      Date    :issued_on,     null: false
+      Date    :due_on,        null: false
+      DateTime :paid_at
+      String  :pdf_key,       null: false
+      String  :notes,         text: true
+      DateTime :created_at,   null: false
+    end
+
+    run "CREATE INDEX invoices_created_at_idx ON invoices (created_at DESC)"
+  end
+end