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>
