Rename Installation to Instance

Follows the portal: the noun is "instance" everywhere the app says it —
UI strings, CLI output, error details, docs and comments. The wire keys
(`instance_id`, `instance_token`) and the hub route this calls move with it.

An existing cloud.json is adopted rather than refused: without the key
alias the dataclass fails to parse, which the caller swallows and reads as
"never enrolled" instead of "reconnect".

`instance_key` on a node type becomes `target_key`. It means the outside
thing a node points at, which is a different sense of the word, and keeping
both would put two meanings of "instance" in one codebase.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015YrQnKV3bnQd4K342y8tKj

backend/fluksio/flow/alerts.py

@@ -358,7 +358,7 @@ class AlertManager:
         await asyncio.to_thread(self.publish, message, alert.model_dump())
 
     async def _send_webpush(self, alert: Alert) -> None:
-        """Wake every browser that subscribed to this installation.
+        """Wake every browser that subscribed to this instance.
 
         No settings of its own: a browser subscribes by pressing a button on
         the alerts screen, and this channel goes to whichever ones did. Saying