docs(migrations): address review comments on alembic config and package layouts

- Drop redundant prepend_sys_path from alembic.ini (env.py handles sys.path)
- Pin db.sqlite path in alembic.ini using %(here)s to avoid cwd dependency
- Add missing __init__.py files in multi-app package layout
- Add noqa: E402 annotations to env.py snippets after sys.path manipulation

Author: pctablet505

docs/models/migrations.md

@@ -157,9 +157,7 @@ class Book(ormar.Model):
 [alembic]
 script_location = %(here)s/alembic
 
-prepend_sys_path = .
-
-sqlalchemy.url = sqlite:///db.sqlite
+sqlalchemy.url = sqlite:///%(here)s/db.sqlite
 ```
 
 `alembic/env.py`:
@@ -180,7 +178,7 @@ if config.config_file_name is not None:
     fileConfig(config.config_file_name)
 
 # Import the shared metadata (and models, so the metaclass registers the tables).
-from my_project.models import metadata
+from my_project.models import metadata  # noqa: E402
 
 target_metadata = metadata
 
@@ -246,8 +244,10 @@ my_project/
 │   ├── models/
 │   │   └── __init__.py
 │   ├── authors/
+│   │   ├── __init__.py
 │   │   └── models.py
 │   └── books/
+│       ├── __init__.py
 │       └── models.py
 └── alembic.ini
 ```
@@ -316,7 +316,7 @@ __all__ = ["Author", "Book", "metadata"]
 Then in `alembic/env.py`:
 
 ```python
-from my_project.models import metadata, Author, Book  # noqa: F401
+from my_project.models import Author, Book, metadata  # noqa: E402, F401
 
 target_metadata = metadata
 ```