BUG: Orphaned Rows in image_embeddings and semantic_labels on Image Deletion

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

Files affected:

- `backend/app/database/image_embeddings.py`
- `backend/app/database/semantic_labels.py`

What is the problem?
Both database modules define foreign key constraints with `ON DELETE CASCADE` referencing the `images(id)` column. This is meant to ensure that when an image is deleted, its corresponding embeddings and semantic labels are deleted automatically.
However, both modules initialize database connections using `_connect()` from `app.database.images` which does not execute `PRAGMA foreign_keys = ON;`. Because SQLite disables foreign keys by default, deleting an image leaves **orphaned embeddings and semantic labels** in the database forever.

Proposed Fix:
Refactor both modules to use the `get_db_connection()` context manager from `app.database.connection` which enforces foreign key constraints correctly.

I wish to work on this issue!

### Record

- [x] I agree to follow this project's Code of Conduct

Read backend/app/database/image_embeddings.py and backend/app/database/semantic_labels.py, then inspect get_db_connection() in app.database.connection and the current _connect() usage from app.database.images. Replace the connection approach as described and verify that deleting an image also removes its embeddings and semantic labels without leaving orphaned rows.

索引モデルが issue の本文から書いたものです。