fix(email): create missing email template and decouple email destination settings

- Create app/templates/email/default.html (fixes 'default.html not found' error)
- Add DEST_EMAIL_* settings to app/config.py (decoupled from shared EMAIL_* settings)
- Update upload_to_email task to use dest_email_* settings exclusively
- Update _should_upload_to_email() to check dest_email_* settings
- Update config validator, providers, and settings_service for dest_email_*
- Update .env.demo and docs/ConfigurationGuide.md
- Update all tests to use dest_email_* settings where appropriate"

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/config.py

@@ -432,7 +432,7 @@ class Settings(BaseSettings):
     # In development/testing, set to True to disable verification (not recommended)
     sftp_disable_host_key_verification: bool = False  # Default enforces host key verification
 
-    # Email settings
+    # Email settings (shared SMTP – used for password reset, verification emails, etc.)
     email_host: Optional[str] = None
     email_port: Optional[int] = 587
     email_username: Optional[str] = None
@@ -441,6 +441,15 @@ class Settings(BaseSettings):
     email_sender: Optional[str] = None  # From address, defaults to email_username if not set
     email_default_recipient: Optional[str] = None
 
+    # Email destination settings (dedicated SMTP for document delivery – decoupled from shared email above)
+    dest_email_host: Optional[str] = None
+    dest_email_port: Optional[int] = 587
+    dest_email_username: Optional[str] = None
+    dest_email_password: Optional[str] = None
+    dest_email_use_tls: bool = True
+    dest_email_sender: Optional[str] = None  # From address for delivered documents
+    dest_email_default_recipient: Optional[str] = None  # Fallback recipient for document delivery
+
     # OneDrive settings
     onedrive_client_id: Optional[str] = None
     onedrive_client_secret: Optional[str] = None