perf(actions): index action_run.commit_sha (#39559)

The API filter `head_sha` on `GET /repos/{owner}/{repo}/actions/runs`
selects runs by `commit_sha`, so `commit_sha` needs an index. For a
action_run table with 212k rows:

- Without the index: the query read 212k rows, and the API request took
35-52 s.
- With the index: the query read 94 rows in 0.14 s, and the API request
took 2-4 s.

---------

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

modelmigration/migrations.go

@@ -429,6 +429,9 @@ func prepareMigrationTasks() []*migration {
 		newMigration(353, "Add audit event table", v28.AddAuditEventTable),
 		newMigration(354, "Add Actions job queue indexes", v28.AddActionQueueIndexes),
 		newMigration(355, "Add AutoMerge merged_commit_id column", v28.AddAutoMergeMergedCommitID),
+		// Gitea 28.0.0 ends at migration ID number 355 (database version 356)
+
+		newMigration(356, "Add index on action_run commit_sha", v28.AddActionRunCommitSHAIndex),
 	}
 	return preparedMigrations
 }