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

roles/web-app-matrix/templates/mautrix/imessage.config.yml.j2

@@ -6,7 +6,7 @@ homeserver:
     # Only the /_matrix/client/unstable/fi.mau.as_sync websocket endpoint is used on this address.
     #
     # Set to null to disable using the websocket. When not using the websocket, make sure hostname and port are set in the appservice section.
-    websocket_proxy: wss://synapse:8008
+    websocket_proxy: ws://synapse:8008
     # How often should the websocket be pinged? Pinging will be disabled if this is zero.
     ping_interval_seconds: 0
     # The domain of the homeserver (also known as server_name, used for MXIDs, etc).