feat(19.3-01): add cooler.Cache with otter memory and Redis drivers

- Cache: Get, Set, Has, Forget, Remember (degrades on cache errors), Increment, Flush
  with per-entry TTLs under <prefix>cache:
- memory driver on otter v2.3.0 bounded by cache.memory.max_entries; Redis driver
  with SET PX, INCRBY and a Flush limited to SCAN MATCH <prefix>cache:* plus UNLINK
- Install publishes the Cache (sharing one Redis client with the rate limiter);
  FromApp serves jobs and commands without a dial
- new docs/services/caching.md, configuration rows, cooler README

docs/setup/configuration.md

@@ -54,7 +54,8 @@ These are the keys most applications set. Each module's reference page lists all
 | `http.body_limits.default_bytes`, `http.body_limits.upload_bytes` | Request body limits. Required by `serve` and `route:list`. | [surf](../../modules/surf/README.md) |
 | `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) |
-| `redis.url` | Redis URL (`redis://`, `rediss://` or `unix://`), required when a Redis store is selected. Keep it in `SUMMER_REDIS__URL` when it carries a password. | [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) |
+| `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://` or `mem://`. Required by `serve`. | [lagoon](../../modules/lagoon/README.md) |
 | `storage.uploads.serve` | Mounts `attach.StaticHandlerPublic` under `storage.uploads.public_path_prefix` with `X-Content-Type-Options: nosniff`, a sandboxing CSP and `Content-Disposition: attachment` for anything but a raster image. Defaults to `false`. | [surf](../../modules/surf/README.md) |