Adds `created_unix` as the third column of the `c_u` composite index on
the `action` table, changing it from `(user_id, is_deleted)` to
`(user_id, is_deleted, created_unix)`.
Migration 337 drops and recreates the index. No data is touched.
## Root causes
#32333 introduced the `c_u` index to speed up dashboard queries, but
defined it as `(user_id, is_deleted)` — without `created_unix`.
#3368 The simple query is now efficient enough for the database to
actually use `c_u`, but because `created_unix` is absent from the index,
the database must load and sort **every** matching row before returning
the first page of 20.
The existing `c_u_d` index `(created_unix, user_id, is_deleted)` does
not help because its leading column is `created_unix`, which can't be
used for an equality seek on `user_id`.
Those two caused this issue:
https://github.com/go-gitea/gitea/issues/38075
With the fix, the database seeks directly to `(user_id=X,
is_deleted=false)` and walks `created_unix` in descending order,
stopping after 20 rows.
Fixes https://github.com/go-gitea/gitea/issues/38075
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