groupware: add /bootstrap

* add a GET /accounts/{a}/boostrap URI that delivers the same as GET /
   but also mailboxes for a given account, in case the UI remembers the
   last used account identifier, to avoid an additional roundtrip

 * streamline the use of simpleError()

 * add logging of errors at the calling site

 * add logging of evictions of Sessions from the cache

 * change default Session cache TTL to 5min instead of 30sec

pkg/structs/structs.go

@@ -2,6 +2,9 @@
 package structs
 
 import (
+	"maps"
+	"slices"
+
 	orderedmap "github.com/wk8/go-ordered-map"
 )
 
@@ -30,3 +33,11 @@ func Uniq[T comparable](source []T) []T {
 	}
 	return set
 }
+
+func Keys[K comparable, V any](source map[K]V) []K {
+	if source == nil {
+		var zero []K
+		return zero
+	}
+	return slices.Collect(maps.Keys(source))
+}