Change: Email notifications for task assignment

Why

Members only discover newly assigned tasks when they happen to open Tally, so assignments sit unnoticed for days. Now that every request has a real user (add-user-auth), we can tell people when work lands on their plate.

What Changes

• Send an email to the assignee whenever a task is assigned or reassigned to them
• Let users opt out of notification emails per project
• Assignment now has a side effect, so the task-management assignment requirement gains a notification scenario

Capabilities

New Capabilities

• notifications: Outbound email notifications and per-project preferences

Modified Capabilities

• task-management: Task assignment now triggers an assignee notification

Impact

• New notification_preferences table
• New background worker cmd/notifier reading from the outbox table
• SMTP credentials added to deployment secrets