Refactor local settings to dotenv-based gallery paths

CLAUDE.md

@@ -53,7 +53,7 @@ The project has one Django app: **`viewer`**. All gallery functionality is conso
 ### Gallery Data Flow
 
 **Filesystem as Data Source:**
-- Images served from `GALLERY_ROOT` path (configured in `local_settings.py`)
+- Images served from `GALLERY_ROOT` path (configured in `.env`)
 - Thumbnails cached in `THUMBNAILS_ROOT` path
 - No database models for images or directories
 - Directory structure navigated using Python's `pathlib`
@@ -108,22 +108,17 @@ Uses Django's built-in `django.contrib.auth` system:
 
 ## Configuration
 
-### Required Local Settings
+### Required Environment Variables
 
-Create `NibasaViewer/local_settings.py` (not in git):
+Create `.env` (not in git):
 
-```python
-from pathlib import Path
-
-GALLERY_ROOT = Path('/path/to/your/images')
-THUMBNAILS_ROOT = Path('/path/to/thumbnail/cache')
-
-# Production settings
-DEBUG = False
-ALLOWED_HOSTS = ['yourdomain.com']
-SECRET_KEY = 'your-secret-key'
+```env
+GALLERY_ROOT=/path/to/your/images
+THUMBNAILS_ROOT=/path/to/thumbnail/cache
 ```
 
+`settings.py` loads `.env` with `python-dotenv` and parses both values as `pathlib.Path`. Each variable is optional and resolves to `None` when unset or empty.
+
 ### Pagination Constants
 
 Defined in viewer/views.py:23-25:
@@ -143,7 +138,7 @@ Default production location: `/var/lib/NibasaViewer`
 
 - **Django 4.2.3** - Web framework
 - **Pillow 10.0.0** - Image processing (thumbnails)
-- **filetype 1.2.0** - Legacy dependency (no longer actively used)
+- **python-dotenv 1.1.1** - Loads local `.env` configuration
 - **gunicorn 21.2.0** - WSGI server (production)
 
 ## Code Patterns