perf: extend action c_u index to include created_unix for faster dashboard feeds (#38076)

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

models/migrations/migrations.go

@@ -416,6 +416,7 @@ func prepareMigrationTasks() []*migration {
 		newMigration(336, "Add ActionRunJobSummary table", v1_27.AddActionRunJobSummaryTable),
 		newMigration(337, "Add visibility to team", v1_27.AddVisibilityToTeam),
 		newMigration(338, "Expand legacy MSSQL issue/comment long-text columns", v1_27.ExpandIssueAndCommentLongTextFieldsForMSSQL),
+		newMigration(339, "Extend action c_u index to include created_unix for faster dashboard feed queries", v1_27.AddCreatedUnixToActionUserIsDeletedIndex),
 	}
 	return preparedMigrations
 }