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

modules/cooler/example_test.go

@@ -0,0 +1,42 @@
+package cooler_test
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/cooler"
+)
+
+func ExampleFromApp() {
+	// serve publishes the Cache at start-up; a job or console command asks
+	// FromApp, which builds the configured driver (memory here) once.
+	app := backpack.New(nil)
+	cache, err := cooler.FromApp(app)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	ctx := context.Background()
+	loads := 0
+	popular := func(context.Context) ([]byte, error) {
+		loads++
+		return json.Marshal([]string{"Hello", "Summer"})
+	}
+	var titles []string
+	for range 2 {
+		raw, err := cache.Remember(ctx, "blog:popular", 10*time.Minute, popular)
+		if err != nil {
+			fmt.Println(err)
+			return
+		}
+		if err := json.Unmarshal(raw, &titles); err != nil {
+			fmt.Println(err)
+			return
+		}
+	}
+	fmt.Println(titles, loads)
+	// Output: [Hello Summer] 1
+}