Remove redundant ix_FileTrash_repo_id index (covered by composite index in seafevents#606)

[yaoge123]

Summary

Add a composite index (repo_id, delete_time) on the FileTrash table and remove the now-redundant single-column repo_id index.

This is the companion PR to haiwen/seafevents#606, which adds the same composite index in the SQLAlchemy model.

Problem

When listing a repository's trash, the query pattern is:

SELECT * FROM FileTrash
WHERE repo_id = ? AND delete_time > ?
ORDER BY delete_time DESC
LIMIT ...

The single-column repo_id index is not optimal for this query. A composite index (repo_id, delete_time) allows the query to locate the target repository's records and walk the index in sorted order, avoiding scanning unnecessary rows and extra sorting.

Tested on a production instance, query time improved from hundreds of milliseconds to under 1 ms.

Changes

1. seahub/base/models.py - Remove db_index=True from FileTrash.repo_id (the single-column index is now redundant)
2. sql/mysql.sql - Replace ix_FileTrash_repo_id (repo_id) with idx_filetrash_repo_delete_time (repo_id, delete_time) in the table definition
3. scripts/upgrade/sql/13.0.0/mysql/seahub.sql - Add ALTER TABLE to create the composite index and DROP INDEX to remove the old one for existing installations