feat(setting): add shared [redis] section as default for redis-backed subsystems (#38550)

Adds a shared `[redis]` config section with a single `CONN_STR` key that
acts as the default Redis connection for every redis-backed subsystem
whose own connection string is empty: `[cache].HOST`,
`[session].PROVIDER_CONFIG`, `[queue].CONN_STR` and
`[global_lock].SERVICE_CONN_STR`.

Precedence: per-subsystem value > shared `[redis].CONN_STR` > existing
built-in defaults. Fully backward compatible: when `[redis]` is not set,
every code path behaves exactly as before.

Motivation: today an admin with one Redis instance has to repeat the
same connection string in up to four places (five once WebSocket lands).
Requested by @wxiaoguang in [this review
discussion](https://github.com/go-gitea/gitea/pull/36965#discussion_r3613306976)
as a prerequisite for #36965. This follows the pattern used by GitLab
and Mastodon: one global Redis definition, per-subsystem overrides fall
back to it.

Since #12385 all identical connection strings already share one client
via `nosql.Manager`, so pointing all subsystems at `[redis].CONN_STR`
naturally converges onto a single shared connection pool.

Covered by table-driven unit tests per subsystem (fallback applies, own
value wins, absent `[redis]` = unchanged behavior). A config-cheat-sheet
update for gitea/docs will follow once this is merged.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>

modules/setting/cache.go

@@ -4,7 +4,6 @@
 package setting
 
 import (
-	"strings"
 	"time"
 
 	"gitea.dev/modules/log"
@@ -14,7 +13,7 @@ import (
 type Cache struct {
 	Adapter  string
 	Interval int
-	Conn     string
+	Conn     string        `ini:"-"`
 	TTL      time.Duration `ini:"ITEM_TTL"`
 }
 
@@ -53,13 +52,12 @@ func loadCacheFrom(rootCfg ConfigProvider) {
 	CacheService.Adapter = sec.Key("ADAPTER").In("memory", []string{"memory", "redis", "memcache", "twoqueue"})
 	switch CacheService.Adapter {
 	case "memory":
-	case "redis", "memcache":
-		CacheService.Conn = strings.Trim(sec.Key("HOST").String(), "\" ")
+	case "memcache":
+		CacheService.Conn = sec.Key("HOST").String()
+	case "redis":
+		CacheService.Conn = sec.Key("HOST").MustString(Redis.ConnStr)
 	case "twoqueue":
-		CacheService.Conn = strings.TrimSpace(sec.Key("HOST").String())
-		if CacheService.Conn == "" {
-			CacheService.Conn = "50000"
-		}
+		CacheService.Conn = sec.Key("HOST").MustString("50000")
 	default:
 		log.Fatal("Unknown cache adapter: %s", CacheService.Adapter)
 	}