feat(19.3-04): cooler site-cache purge service with site_cache.purge_url and purge_token

- cooler.SiteCachePurger moved from the Journal plugin: loopback-only http,
  32-character token, 2 s detached request, no token in logs or errors
- NewSiteCachePurgerFor names caller keys (for plugin alias keys)
- PurgeSiteCacheAfterCommit through lagoon.AfterCommit, one POST per save
- SiteCacheFromApp for hooks outside serve; Install publishes the purger
  unless a plugin already did, and fails boot on an invalid config
- caching docs section, cooler README, configuration row

README.md

@@ -99,7 +99,7 @@ The `migrate`, `migrate:status`, `migrate:rollback`, `serve` and admin commands
 | [cabana](modules/cabana/README.md) | Schema-driven admin backend that compiles WinterCMS-style YAML list, form, filter and relation definitions at boot and serves them as a JSON admin API next to the embedded admin SPA. | The beach hut where the staff run the beach; it is the admin backend. |
 | [compass](modules/compass/README.md) | Layered YAML configuration with per-environment directories, `SUMMER_` environment overrides, embedded plugin defaults and dot-path access. | Points you the right way; configuration tells the application how to behave in each environment. |
 | [conga](modules/conga/README.md) | Background jobs on River over the shared Postgres pool: transactional dispatch, a `summer_jobs` progress record, in-process or dedicated workers, and a wall-clock scheduler. | A line of dancers moving one after another; jobs queue up and are worked through in order. |
-| [cooler](modules/cooler/README.md) | Cache drivers and shared counters for SummerCMS: memory and Redis caches with a key prefix and TTLs, and a Redis-backed rate-limit store. | The cool box you bring to the beach; it keeps things fresh and at hand for everyone in the group. |
+| [cooler](modules/cooler/README.md) | Cache drivers and shared counters for SummerCMS: memory and Redis caches with a key prefix and TTLs, a Redis-backed rate-limit store, and the site-cache purge after admin saves. | The cool box you bring to the beach; it keeps things fresh and at hand for everyone in the group. |
 | [festival](modules/festival/README.md) | Typed, synchronous event bus with listener priorities, payload collection and stop-when-handled dispatch. | A summer festival announces events and the crowd reacts; it is the event bus. |
 | [fetchguard](modules/fetchguard/README.md) | Guarded outbound HTTP client and fetcher that blocks private and reserved addresses, enforces host, size and timeout limits, and offers an explicit trusted mode for operator-configured endpoints. | A lifeguard for outbound fetches; it watches every request the application sends out and stops the dangerous ones. |
 | [flare](modules/flare/README.md) | Web Push delivery with VAPID (RFC 8292) and aes128gcm payload encryption (RFC 8291) behind a small Pusher interface. | A signal flare reaches people far away; it sends Web Push notifications to browsers. |