db: issuer_profiles + clients migration

issuer_profiles is one row per user with all the legal-entity fields
that previously lived in the ISSUER constant — legal name, tax id,
reg number, bank details — plus per-user defaults (invoice prefix,
currency, due days, notes). On invoice creation these get snapshotted
onto the invoice row so historical PDFs render unchanged when the
user later edits their profile.

clients is per-user, case-insensitive unique on (owner_id, email).
Partial index on active clients for the dropdown. Soft-archive only;
ON DELETE RESTRICT to honor the indefinite-retention rule.

db/migrations/003_issuer_profiles_and_clients.rb

@@ -0,0 +1,41 @@
+Sequel.migration do
+  change do
+    create_table(:issuer_profiles) do
+      foreign_key :user_id, :users, type: :Bignum, primary_key: true,
+                  on_delete: :restrict
+      String   :legal_name
+      String   :legal_name_local
+      String   :tax_id
+      String   :reg_number
+      Date     :registered_on
+      String   :address,          text: true
+      String   :contact_email
+      String   :contact_phone
+      String   :bank_name
+      String   :bank_iban
+      String   :bank_swift
+      String   :invoice_prefix,   null: false, default: 'INV'
+      String   :default_currency, null: false, default: 'USD', size: 3
+      Integer  :default_due_days, null: false, default: 14
+      String   :default_notes,    text: true
+      DateTime :updated_at,       null: false
+    end
+
+    create_table(:clients) do
+      primary_key :id, type: :Bignum
+      foreign_key :owner_id, :users, type: :Bignum, null: false,
+                  on_delete: :restrict
+      String   :name,            null: false
+      String   :email,           null: false
+      String   :address,         text: true
+      String   :default_currency, size: 3
+      Integer  :default_due_days
+      String   :default_notes,   text: true
+      DateTime :archived_at
+      DateTime :created_at,      null: false
+    end
+
+    run "CREATE UNIQUE INDEX clients_owner_email_idx ON clients (owner_id, lower(email))"
+    run "CREATE INDEX clients_owner_active_idx ON clients (owner_id) WHERE archived_at IS NULL"
+  end
+end