feat: restore worker recycle limit via Caddyfile max_requests (default 500)

The removed runtime/frankenphp-symfony package force-restarted the
worker after FRANKENPHP_LOOP_MAX requests (default 500) - memory-leak
paranoia the built-in 8.1 runner intentionally doesn't reimplement.

Restore the behavior natively so the upgrade is a no-op operationally:
- Caddyfile: global frankenphp block with max_requests {$MAX_REQUESTS}
  - per-thread graceful restarts, other threads keep serving
- Dockerfile: ARG/ENV MAX_REQUESTS=500 bakes the same default the old
  package had, overridable at docker build or runtime (-e, 0 disables)
- Stock FRANKENPHP_CONFIG env var remains the full-config escape hatch
- Documented in readme env table, docs/example.env, CHANGELOG, and the
  upgrade plan (phase-4 staging note now checks thread recycling)

docs/example.env

@@ -24,6 +24,14 @@
 # once blocked, do we respond with "I'm a teapot", false to use "Too many requests"
 #TEAPOT=true                # default enabled, boolean
 
+# --- server / worker options ---
+
+# (container/deployment only) restart each FrankenPHP worker thread after
+# this many requests, containing memory growth across long uptime;
+# matching the default from the old runtime/frankenphp-symfony package.
+# 0 disables restarts. consumed by the Caddyfile, not the PHP app.
+#MAX_REQUESTS=500           # default 500
+
 # --- remote-user header ---
 # Controls the value sent in the Remote-User header on successful auth.
 # session: the session id (default, backward-compatible)