Item command: support multiple groups

Support items being added to multiple groups. When present:

• All groups assigned have "received" count incremented
• Highest "resolved" priority is used: Ignore (see #56 (closed)) > Custom > High > Medium > Low > Default

Use existing item resolution - it already records the full "chain", group precedence, and notification flag.

When items are added to / removed from a group, add / subtract receive counts as before.