Evict archive writers on deletion and sweep idle archives (closes #89) (#95)

Per-webhook archive writers are now evicted when the webhook or its last
database target is deleted, and a background sweeper prunes expired rows from
idle archives that no longer receive writes. Archive files themselves are never
deleted.

internal/delivery/target.go

@@ -90,12 +90,15 @@ func (e *Engine) initTargets(client *http.Client) {
 		client:   client,
 	}
 
+	dbT := &databaseTarget{eng: e}
+
 	e.httpTarget = httpT
+	e.dbTarget = dbT
 
 	e.targets = map[database.TargetType]Target{
 		database.TargetTypeHTTP:     httpT,
 		database.TargetTypeSlack:    slackT,
-		database.TargetTypeDatabase: &databaseTarget{eng: e},
+		database.TargetTypeDatabase: dbT,
 		database.TargetTypeLog:      &logTarget{eng: e},
 	}
 }