fix: enable file re-indexing in local mode watcher

[echobt]

Description

This PR fixes a bug where the file watcher in Local mode would detect file changes but never actually re-index them. It only printed a "(pending)" message.

Changes

• Implemented index_file_local in watcher.rs to handle re-indexing using the local embedding engine.
• Updated process_files to call index_file_local when in Mode::Local, ensuring files are updated in the database.
• Removed the placeholder "(pending)" log message and replaced it with a success message upon indexing.

Verification

• Verified that file changes in Local mode now trigger re-indexing.
• cargo test passes.