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

docs/setup/configuration.md

@@ -55,6 +55,7 @@ These are the keys most applications set. Each module's reference page lists all
 | `http.cors.*`, `http.trusted_proxies` | CORS and the proxies whose forwarded client IP is trusted. | [surf](../../modules/surf/README.md) |
 | `http.rate_limit.store` | Where `throttle` counters live: `memory` (per instance, the default) or `redis` (shared by every instance). | [cooler](../../modules/cooler/README.md) |
 | `cache.driver`, `cache.prefix`, `cache.memory.max_entries` | The cache driver (`memory`, the default, or `redis`), the key prefix (default from `app.name`) and the memory driver's size bound (default 10000). | [cooler](../../modules/cooler/README.md) |
+| `site_cache.purge_url`, `site_cache.purge_token` | The host site's page-cache purge route and its bearer token (at least 32 characters). Set both or neither; http only to a loopback host. Keep the token in `SUMMER_SITE_CACHE__PURGE_TOKEN`. | [cooler](../../modules/cooler/README.md) |
 | `redis.url` | Redis URL (`redis://`, `rediss://` or `unix://`), required when a Redis cache or rate-limit store is selected. Keep it in `SUMMER_REDIS__URL` when it carries a password. | [cooler](../../modules/cooler/README.md) |
 | `http.access_log` | Logs one record per request with method, matched route pattern, status and duration. Defaults to `false`. | [surf](../../modules/surf/README.md) |
 | `storage.uploads.bucket_url` | Uploads bucket, `file://`, `mem://` or `s3://` (the last only in a binary built with the `summercms_s3` tag from `build.tags` in `summer.yaml`; credentials from the AWS environment). Required by `serve`. | [lagoon](../../modules/lagoon/README.md) |