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>

helpers/CacheHelper.php

@@ -125,16 +125,23 @@ class CacheHelper
             return !file_exists($filePath) || @unlink($filePath);
         }
 
-        // Delete all files with this prefix
-        $pattern = self::getCacheDir() . '/' . preg_replace('/[^a-zA-Z0-9_]/', '_', $prefix) . '*.json';
-        $files = glob($pattern);
+        // Delete all entries for this prefix. A key is either the bare prefix or
+        // prefix + '_' + md5(identifier) (32 hex chars, see makeKey). Match exactly
+        // that so a prefix can't clobber a different prefix that merely shares a
+        // leading substring — e.g. delete('workflow') must not wipe 'workflow_rules'.
+        $safePrefix = preg_replace('/[^a-zA-Z0-9_]/', '_', $prefix);
+        $keyRegex   = '/^' . preg_quote($safePrefix, '/') . '(_[0-9a-f]{32})?$/';
+
+        $files = glob(self::getCacheDir() . '/' . $safePrefix . '*.json') ?: [];
         foreach ($files as $file) {
-            @unlink($file);
+            if (preg_match($keyRegex, basename($file, '.json'))) {
+                @unlink($file);
+            }
         }
 
-        // Clear memory cache entries with this prefix
+        // Clear matching memory cache entries
         foreach (array_keys(self::$memoryCache) as $key) {
-            if (strpos($key, $prefix) === 0) {
+            if (preg_match($keyRegex, $key)) {
                 unset(self::$memoryCache[$key]);
             }
         }