db: scope invoices to owner, add void + snapshot columns

- owner_id (FK users, ON DELETE RESTRICT) + index on (owner_id, created_at DESC)
- client_id (nullable FK clients, ON DELETE RESTRICT) for client back-references
- issuer_snapshot jsonb + snapshot_version: invoices freeze the issuer's
  legal-entity fields at issue time so historical PDFs render unchanged when
  the user later edits their profile
- voided_at + voided_reason: soft-void instead of delete, since invoices
  are retained indefinitely
- swap unique(number) for unique(owner_id, number): numbers are now per-user,
  not global

DROP CONSTRAINT uses IF EXISTS so the migration is resilient to the
auto-generated constraint name differing across Postgres versions.

db/migrations/004_scope_invoices_to_owner.rb

@@ -0,0 +1,23 @@
+Sequel.migration do
+  change do
+    alter_table(:invoices) do
+      add_foreign_key :owner_id,  :users,   type: :Bignum, on_delete: :restrict
+      add_foreign_key :client_id, :clients, type: :Bignum, on_delete: :restrict
+      add_column :issuer_snapshot,  :jsonb
+      add_column :snapshot_version, Integer, null: false, default: 1
+      add_column :voided_at,        DateTime
+      add_column :voided_reason,    String, text: true
+
+      add_unique_constraint [:owner_id, :number], name: :invoices_owner_number_uniq
+    end
+
+    # Drop the original global-unique constraint on (number). Postgres names
+    # unique constraints from `unique: true` as `<table>_<column>_key`. Use raw
+    # SQL with IF EXISTS so this no-ops if the name ever differs.
+    run "ALTER TABLE invoices DROP CONSTRAINT IF EXISTS invoices_number_key"
+
+    alter_table(:invoices) do
+      add_index [:owner_id, Sequel.desc(:created_at)], name: :invoices_owner_created_idx
+    end
+  end
+end