Add invoice bank account templates API and migration.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/097_invoice_account_templates.sql

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+-- Reusable bank account templates (platform or business), parallel to invoice_item_templates.
+
+CREATE TABLE IF NOT EXISTS invoice_account_templates (
+  id BIGSERIAL PRIMARY KEY,
+  owner_scope invoice_owner_scope NOT NULL,
+  business_id BIGINT NULL REFERENCES businesses (id) ON DELETE CASCADE ON UPDATE NO ACTION,
+  bank_name VARCHAR(255) NOT NULL,
+  account_holder_name VARCHAR(255) NULL,
+  card_number VARCHAR(64) NULL,
+  iban VARCHAR(64) NULL,
+  sort_order INT NOT NULL DEFAULT 0,
+  is_active BOOLEAN NOT NULL DEFAULT TRUE,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_invoice_account_templates_owner_scope
+  ON invoice_account_templates (owner_scope, sort_order);
+
+CREATE INDEX IF NOT EXISTS idx_invoice_account_templates_business_id
+  ON invoice_account_templates (business_id);
+
+DROP TRIGGER IF EXISTS invoice_account_templates_set_updated_at ON invoice_account_templates;
+CREATE TRIGGER invoice_account_templates_set_updated_at
+  BEFORE UPDATE ON invoice_account_templates
+  FOR EACH ROW
+  EXECUTE FUNCTION set_updated_at();