Play Framework

fix: performance — EhCacheImpl desync, TemplateLoader TOCTOU, JobsPlugin volatile fields

Cherry-picked from worktree-agent-ae649d40 (fc7b19eb7), with the following
applied:
- EhCacheImpl.java: remove synchronized from instance methods (EhCache is
  thread-safe internally; synchronized was pinning virtual threads)
- TemplateLoader.java: fix check-then-act race on ConcurrentHashMap using
  compute/computeIfAbsent
- JobsPlugin.java: add volatile to executor, virtualExecutor,
  usingVirtualThreads fields

Deliberately not applied from the original commit:
- VirtualThreadScheduledExecutor.java: the original swapped the scheduler's
  ThreadFactory from PThreadFactory to Thread.ofVirtual().factory(). This
  contradicts the file's documented two-tier design ("delegates scheduling
  to a small platform-thread ScheduledThreadPoolExecutor ... dispatches
  actual work to virtual threads") which exists specifically to avoid
  pinning a carrier thread on DelayQueue.take(). Kept the platform-thread
  scheduler.
- JobsPlugin.scheduledJobs: original commit's tree had CopyOnWriteArrayList
  (introduced in earlier modernization commit 264612b1d, not in this branch).
  Kept ArrayList here; CopyOnWriteArrayList migration is its own decision.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>