v0.4.7

Replaces the singleton SMTP settings page with per-tenant sender accounts.

• New MailSender model (communication_mail_senders): a tenant can keep any number of sender accounts, exactly one flagged as default, each with its own from/reply-to address and SMTP credentials (password stored encrypted).
• List and detail components plus YAML configs under /mail-senders; the old /communication-settings and /marketing-settings routes redirect there.
• Existing custom SMTP settings are migrated into a default sender per tenant, after which the SMTP and reply_email columns are dropped from communication_settings.
• CommunicationSetting resolves the from and reply-to address from the tenant's default sender; TenantSmtpResolver and Communicator build the tenant mailer from the sender account. An account without SMTP credentials still carries from/reply and relays through the platform mailer.
• reply_email was never a reply-to — its only readers treated it as the tenant's notification address, which now lives in the liefertool module.