Cacheable process list. Emits a content ETag and honours If-None-Match for 304s, saving the ~9 KB body on every unchanged poll (the SPA polls this every ~1.8 s).

The validator is an ETag, not Last-Modified: a max(updated_at) high-water mark moves backwards when a process is deleted, which produced stale 304s (a phantom process lingering in the client's cached list). See the inline comment below.

Backwards compatible: clients that send no conditional header — or send the old If-Modified-Since (now ignored) — receive the full JSON body with a 200, so they always render the current set.