feat(inbox): add priority/due_date notifications, structured details, and hover card

- Add missing notifications for priority_changed and due_date_changed events
- Publish priority_changed and due_date_changed flags from UpdateIssue handler
- Add details JSONB column to inbox_item (migration 019) for structured change data
- Store from/to values in details for status, priority, assignee, and due_date changes
- Notification titles now use plain issue title; details carry structured context
- Add human-readable label maps (statusLabels, priorityLabels) in notification listeners
- Update inbox handler responses to include details field
- Frontend: InboxDetailLabel renders rich subtitles per notification type
  - Status: "Set status to ● In Progress" with StatusIcon
  - Priority: "Set priority to ◆ High" with PriorityIcon
  - Assigned: "Assigned to Bob" with resolved actor name
  - Due date: "Set due date to Apr 20"
  - Comment: truncated comment body preview
- Frontend: HoverCard on inbox items shows issue title + description context
- Add due_date_changed to InboxItemType and typeLabels
- Add tests for priority_changed and due_date_changed notifications

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/db/queries/inbox.sql

@@ -15,8 +15,8 @@ WHERE id = $1;
 INSERT INTO inbox_item (
     workspace_id, recipient_type, recipient_id,
     type, severity, issue_id, title, body,
-    actor_type, actor_id
-) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10)
+    actor_type, actor_id, details
+) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11)
 RETURNING *;
 
 -- name: MarkInboxRead :one