fix

The current custom template only supports {chat_id}, {int_ts}, {ts}, and {key}, all of which assume a Telegram-shaped message. For Home Assistant apache_kafka messages (and likely other sources), there's no top-level timestamp field — the time lives in last_updated / last_changed, and unique IDs live nested at context.id.
This change:

Adds {last_updated}, {last_changed}, and {entity_id} as first-class tokens.
Adds dotted-path resolution to field mode so COUCHDB_ID_FIELD=context.id works.
Adds a generic {doc.path.to.field} escape hatch in custom templates so users can reference any nested field without further code changes.
Sanitizes characters in template values that would break CouchDB _ids (specifically /).

Fully backward-compatible — existing templates and modes behave identically.