feat: enable environment variable overrides for static and media roots

Update STATIC_ROOT and MEDIA_ROOT in settings.py to read from
environment variables with default fallbacks to BASE_DIR paths.
This allows flexible deployment configurations without modifying
source code for different environments.

mnemosyne/mnemosyne/settings.py

@@ -203,9 +203,9 @@ USE_TZ = True
 
 # --- Static files ---
 STATIC_URL = "static/"
-STATIC_ROOT = BASE_DIR / "staticfiles"
+STATIC_ROOT = Path(env("STATIC_ROOT", default=str(BASE_DIR / "staticfiles")))
 
-MEDIA_ROOT = BASE_DIR / "media"
+MEDIA_ROOT = Path(env("MEDIA_ROOT", default=str(BASE_DIR / "media")))
 MEDIA_URL = "/media/"
 
 # --- Storage (S3 or local) ---