Fix#8033 Database Join add load SQL from a file

[leehaut]

Fix #8033

Summary

Adds Load SQL from file to the Database Join transform, matching Table Input.

A Database Join can now point at an external .sql file (VFS path, variables such as ${PROJECT_HOME}/sql/lookup_records.sql). The query is read once at transform init. ? placeholders still bind to the Parameters table, so the same lookup SQL can be reused across pipelines without copy-paste.

Leaving the path empty keeps the previous inline SQL editor (backward compatible).

Changed

• Runtime: sql_from_file on DatabaseJoinMeta; getEffectiveSql() loads the file via VFS (UTF-8). DatabaseJoin.init() prepares that SQL, then applies Replace variables when enabled.
• UI: path + Browse above the SQL editor. When a file is set, the editor is a read-only preview; clearing the path makes it editable again.
• Docs / i18n: user manual (databasejoin.adoc) and en_US / zh_CN messages.
• Add integration test 0009-database-join-sql-file.hpl