Address remaining review items: Synapse caching, cycle detection, cache/ratelimit/kanban

- SynapseHelper: memoize username->Matrix-ID lookups per-request (incl. negative
  results) and add an overall time budget to resolveUsernames() plus a 2s connect
  timeout, so notifying N watchers with a slow/unreachable Synapse can't stall the
  request for N x 5s. (Chosen over async/queue per maintainer.)
- DependencyModel: fix cycle detection treating 'blocks' and 'blocked_by' as the
  same edge direction. They are inverse relationships (single row each, no mirror
  row), so the traversal now walks a unified precedence graph (blocks: ticket->
  depends_on; blocked_by: depends_on->ticket) and wouldCreateCycle normalizes the
  new edge's direction. Prevents both false-positive and missed cycles.
- CacheHelper: anchor prefix-delete to exact key boundaries (bare prefix or
  prefix + '_' + md5) so delete('workflow') can't wipe a 'workflow_rules' cache.
- RateLimitMiddleware: hold an exclusive flock across the per-IP counter's
  read-modify-write so concurrent requests can't both read N and write N+1
  (undercounting past the limit). Fails open if the file can't be locked.
- dashboard.js: kanban status update now uses lt.api.post (per no-raw-fetch
  convention) and reverts the card AND the optimistic column counts on failure
  (the old raw-fetch catch left the card moved without reverting).
- BulkOperationsModel: document that bulk_status/bulk_close intentionally bypass
  workflow transition validation (admin override, by design).

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

models/DependencyModel.php

@@ -190,14 +190,25 @@ class DependencyModel
      */
     private function wouldCreateCycle($ticketId, $dependsOnId, $type): bool
     {
-        // Only check for cycles in blocking relationships
+        // Only blocking relationships impose an ordering that can form a cycle.
         if (!in_array($type, ['blocks', 'blocked_by'])) {
             return false;
         }
 
-        // Check if dependsOnId already has ticketId in its dependency chain
+        // Normalize the new row to a precedence edge "from must finish before to":
+        //   (t, d, 'blocks')     => t blocks d      => edge t -> d
+        //   (t, d, 'blocked_by') => t blocked_by d  => edge d -> t
+        if ($type === 'blocks') {
+            $from = $ticketId;
+            $to   = $dependsOnId;
+        } else { // blocked_by
+            $from = $dependsOnId;
+            $to   = $ticketId;
+        }
+
+        // Adding edge from->to creates a cycle iff a path to ->* from already exists.
         $visited = [];
-        return $this->hasDependencyPath($dependsOnId, $ticketId, $visited, 0);
+        return $this->hasDependencyPath($to, $from, $visited, 0);
     }
 
     /**
@@ -236,15 +247,22 @@ class DependencyModel
 
         $visited[] = $source;
 
-        $sql = "SELECT depends_on_id FROM ticket_dependencies
-                WHERE ticket_id = ? AND dependency_type IN ('blocks', 'blocked_by')";
+        // Walk the unified precedence graph forward from $source. Both directions
+        // of expression contribute an outgoing edge "$source must finish before X":
+        //   blocks rows where ticket_id=$source     -> X = depends_on_id
+        //   blocked_by rows where depends_on_id=$source -> X = ticket_id
+        $sql = "SELECT depends_on_id AS next_id FROM ticket_dependencies
+                    WHERE ticket_id = ? AND dependency_type = 'blocks'
+                UNION
+                SELECT ticket_id AS next_id FROM ticket_dependencies
+                    WHERE depends_on_id = ? AND dependency_type = 'blocked_by'";
         $stmt = $this->conn->prepare($sql);
-        $stmt->bind_param("s", $source);
+        $stmt->bind_param("ss", $source, $source);
         $stmt->execute();
         $result = $stmt->get_result();
 
         while ($row = $result->fetch_assoc()) {
-            if ($this->hasDependencyPath($row['depends_on_id'], $target, $visited, $depth + 1)) {
+            if ($this->hasDependencyPath($row['next_id'], $target, $visited, $depth + 1)) {
                 $stmt->close();
                 return true;
             }