feat: Add authentication configuration and validation

- Introduced new authentication settings in config.py including `auth_enabled`, `admin_username`, `admin_password`, and `session_secret`.
- Added validation for `session_secret` to ensure it meets security requirements when authentication is enabled.
- Updated main.py to conditionally mount static files and log warnings if the directory is not found.
- Removed unused email template files and added new authentication and notification setup documentation.
- Implemented authentication configuration validation in validators.py and updated settings display.
- Enhanced the user interface with a new login template and SVG assets for branding.
- Added comprehensive guides for setting up authentication and notifications in the documentation.

.env.demo

@@ -8,7 +8,9 @@ ALLOW_FILE_DELETE=true  # Allow deletion of file records
 
 # **Authentication**
 AUTH_ENABLED=true
-SESSION_SECRET=<atLeast32Characters>
+# Generate a secure random string, for example:
+# python -c "import secrets; print(secrets.token_hex(32))"
+SESSION_SECRET=b39fd43f68d0491ca942f28a16e484b1e763fe9accf4445ca2669a5f3b179eb4
 ADMIN_USERNAME=admin
 ADMIN_PASSWORD=your_secure_password
 
@@ -16,6 +18,7 @@ ADMIN_PASSWORD=your_secure_password
 AUTHENTIK_CLIENT_ID=<yourAuthentikAppClientID>
 AUTHENTIK_CLIENT_SECRET=<yourAuthentikClientSecret>
 AUTHENTIK_CONFIG_URL=<ConfigUrlOfYourApp, e.g. https://authentik.example.com/application/o/docuelevate/.well-known/openid-configuration>
+OAUTH_PROVIDER_NAME="Authentik SSO"
 
 # **AI/ML Services**
 # OpenAI