Fix User Activity report's Tickets Assigned column to use assignment date (#49)

The "Tickets Assigned" column filtered by tickets.created_at, so a ticket
created outside the selected date range but assigned to a user within it
never counted, while one created in-range but assigned/reassigned later
counted as if the assignment happened in-range — filtered by the wrong
date field for what the column claims to measure.

tickets has no assigned_at column, so derive the count from audit_log's
'assign' events (already logged by both the single-ticket and bulk-assign
paths) instead, filtered by the event's own created_at. COUNT(DISTINCT
entity_id) so a ticket reassigned more than once to the same user within
the range still counts once.

Verified against real MariaDB: seeded one ticket created outside the test
range but assigned inside it, and one created inside the range but
assigned outside it. The old query counted the wrong one; the new query
correctly flips to count the one actually assigned within the range.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117oBw2jN4kALYeS8HPq4zV

index.php

@@ -388,9 +388,19 @@ switch (true) {
                     GROUP BY user_id
                 ) cm ON u.user_id = cm.user_id
                 LEFT JOIN (
-                    SELECT assigned_to, COUNT(*) as tickets_assigned
-                    FROM tickets
-                    WHERE DATE(created_at) BETWEEN ? AND ?
+                    -- Assignment date, not ticket creation date: a ticket created
+                    -- outside the range but assigned within it should count, and
+                    -- one created in-range but assigned later shouldn't (until it
+                    -- is). Derived from audit_log's 'assign' events since tickets
+                    -- has no assigned_at column; COUNT(DISTINCT ...) so a ticket
+                    -- reassigned more than once to the same user in-range still
+                    -- counts once.
+                    SELECT
+                        CAST(JSON_UNQUOTE(JSON_EXTRACT(details, '$.assigned_to')) AS UNSIGNED) as assigned_to,
+                        COUNT(DISTINCT entity_id) as tickets_assigned
+                    FROM audit_log
+                    WHERE action_type = 'assign' AND entity_type = 'ticket'
+                        AND DATE(created_at) BETWEEN ? AND ?
                     GROUP BY assigned_to
                 ) ta ON u.user_id = ta.assigned_to
                 LEFT JOIN (