refactor: move go-chi/cache into Gitea (#39530)

The `gitea.com/go-chi/cache` package only exists for Gitea, so it moves
into `modules/cache`.

- `ITEM_TTL = -1` disables caching as documented
- Sub-second TTLs round up instead of never expiring
- The Redis adapter no longer grows a `MacaronCache` hash
- Use two-queue cache for in-memory cache

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/cache/cache_memcache.go

@@ -0,0 +1,70 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cache
+
+import (
+	"errors"
+	"fmt"
+	"math"
+	"strings"
+
+	"github.com/bradfitz/gomemcache/memcache"
+)
+
+const memcacheMaxRelativeTTL = 30 * 24 * 60 * 60
+
+type memcacheCache struct {
+	client *memcache.Client
+}
+
+func newMemcacheCache(conn string) (backend, error) {
+	if conn == "" {
+		return nil, errors.New("cache adapter memcache requires [cache] HOST, e.g. 127.0.0.1:11211")
+	}
+	servers := &memcache.ServerList{}
+	if err := servers.SetServers(strings.Split(conn, ";")...); err != nil {
+		return nil, fmt.Errorf("invalid memcache HOST %q: %w", conn, err)
+	}
+	return &memcacheCache{client: memcache.NewFromSelector(servers)}, nil
+}
+
+// memcacheExpiration converts a TTL beyond memcached's 30-day relative limit into an absolute unix time
+func memcacheExpiration(ttl int64) int32 {
+	if ttl > memcacheMaxRelativeTTL {
+		ttl += timeNow().Unix()
+	}
+	return int32(min(ttl, math.MaxInt32))
+}
+
+func (c *memcacheCache) Get(key string) (string, bool) {
+	item, err := c.client.Get(key)
+	if err != nil {
+		return "", false
+	}
+	return string(item.Value), true
+}
+
+func (c *memcacheCache) Put(key, value string, ttl int64) error {
+	return c.client.Set(&memcache.Item{Key: key, Value: []byte(value), Expiration: memcacheExpiration(ttl)})
+}
+
+func (c *memcacheCache) Delete(key string) error {
+	if err := c.client.Delete(key); err != nil && !errors.Is(err, memcache.ErrCacheMiss) {
+		return err
+	}
+	return nil
+}
+
+func (c *memcacheCache) IsExist(key string) bool {
+	_, err := c.client.Get(key)
+	return err == nil
+}
+
+func (c *memcacheCache) Ping() error {
+	const key = "__gitea_cache_ping"
+	if err := c.Put(key, "ping", 10); err != nil {
+		return err
+	}
+	return c.Delete(key)
+}