Refactor websocket and Taiga variables

- Introduce WEBSOCKET_PROTOCOL derived from WEB_PROTOCOL (wss if https, else ws).
- Replace hardcoded websocket URLs in EspoCRM, Nextcloud and Taiga with {{ WEBSOCKET_PROTOCOL }}.
- Fix mautrix-imessage to use ws:// for internal synapse:8008.
- Standardize Pixelfed OIDC env spacing.
- Refactor Taiga variables to TAIGA_* naming convention and clean up EMAIL_BACKEND definition.

See: https://chatgpt.com/share/68af62fa-4dcc-800f-9aaf-cff746daab1e

group_vars/all/00_general.yml

@@ -26,6 +26,9 @@ HOST_DECIMAL_MARK:        ","
 WEB_PROTOCOL:           "https"       # Web protocol type. Use https or http. If you run local you need to change it to http
 WEB_PORT:               "{{ 443 if WEB_PROTOCOL == 'https' else 80 }}"  # Default port web applications will listen to
 
+# Websocket
+WEBSOCKET_PROTOCOL:     "{{ 'wss' if WEB_PROTOCOL == 'https' else 'ws' }}"
+
 # Domain
 PRIMARY_DOMAIN:         "localhost" # Primary Domain of the server