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/queue.go

@@ -9,6 +9,7 @@ import (
 
 	"gitea.dev/modules/json"
 	"gitea.dev/modules/log"
+	"gitea.dev/modules/util"
 )
 
 // QueueSettings represent the settings for a queue from the ini
@@ -77,8 +78,10 @@ func GetQueueSettings(rootCfg ConfigProvider, name string) (QueueSettings, error
 	}
 	cfg.Datadir = filepath.ToSlash(cfg.Datadir)
 
+	// redis fallback order: [queue]/[queue.name] CONN_STR (already merged above) wins,
+	// then the shared [redis] conn, then the built-in localhost default for compatibility
 	if cfg.Type == "redis" && cfg.ConnStr == "" {
-		cfg.ConnStr = "redis://127.0.0.1:6379/0"
+		cfg.ConnStr = util.IfZero(Redis.ConnStr, "redis://127.0.0.1:6379/0")
 	}
 
 	if cfg.Length <= 0 {