Fix logic bugs found in third multi-agent review

Medium:
- create_ticket_api.php: environment tags were parsed with explode('][') which
  left brackets on the first/last tag so the whitelist never matched, dropping
  the env tag from the dedup hash — a [production] and [staging] issue with
  otherwise-identical components could collide onto one ticket. Use a
  bracket-aware regex.
- CommentModel::getThreadedCommentsPaged only fetched DIRECT children of root
  comments, so when pagination is active, nested replies at depth 2-3 vanished
  from the thread. Expand replies level-by-level (bounded to depth 3).
- StatsModel::getTicketsByAssignee ignored the visibility filter the rest of the
  stats apply, so a non-admin's "by assignee" widget counted (leaked) confidential
  tickets. Thread the same filter through.
- watch_ticket.php GET path returned watch state / watcher names / count for any
  ticket with no access check (the POST path checks it) — added canUserAccessTicket.
- dashboard.js kanban: every card rendered as P4 because the [class*="lt-p"]
  selector never matched the lt-badge-p1 class and the fallback didn't strip "P".
  Extract the digit directly.

Low:
- audit_log.php CSV: "Log ID" column was always blank ($log['log_id'] vs the real
  audit_id column). Use audit_id.
- check_duplicates.php: the graceful-degradation try/catch only covered the throw
  path; guard the false-return (non-exception mysqli) path too.
- notifications.php: owner-who-is-also-@mentioned got two notifications for one
  comment; drop the duplicate comment row when a mention covers the same comment.
- dashboard.js hover preview rendered "PP1" (doubled prefix); strip the leading P.
- markdown.js: code/inline-code restore used string replace, so $&, $$, $`, $' in
  user code were treated as replacement patterns; use a function replacer. Also
  removed an unused loop var.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/js/markdown.js

@@ -142,12 +142,14 @@ function parseMarkdown(markdown) {
     html = html.replace(/  \n/g, '<br>');
     html = html.replace(/\n\n/g, '</p><p>');
 
-    // Restore code blocks and inline code
+    // Restore code blocks and inline code. Use a function replacer so '$'
+    // sequences in user code (e.g. $&, $$, $`, $') are inserted literally rather
+    // than interpreted as String.replace replacement patterns.
     codeBlocks.forEach((block, i) => {
-        html = html.replace('%%CODEBLOCK' + i + '%%', block);
+        html = html.replace('%%CODEBLOCK' + i + '%%', () => block);
     });
     inlineCodes.forEach((code, i) => {
-        html = html.replace('%%INLINECODE' + i + '%%', code);
+        html = html.replace('%%INLINECODE' + i + '%%', () => code);
     });
 
     // Restore footnote reference placeholders
@@ -164,7 +166,7 @@ function parseMarkdown(markdown) {
     // Append footnote definitions block
     if (footnoteOrder.length) {
         html += '<hr class="fn-hr"><ol class="fn-list">';
-        footnoteOrder.forEach(function(label, i) {
+        footnoteOrder.forEach(function(label) {
             html += '<li id="fn-' + fnSlug(label) + '" class="fn-item">' +
                 parseMarkdown(footnotes[label]).replace(/<\/?p>/g, '') +
                 ' <a href="#fnref-' + fnSlug(label) + '" class="fn-back">&#x21A9;</a></li>';