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/api/routes/secrets.py

@@ -13,7 +13,7 @@ from fluksio.api.deps import get_current_active_superuser
 from fluksio.flow.secrets import SecretNotFound, get_secrets
 from fluksio.models import Message
 
-# Superuser, not merely signed in. The names alone say what this installation
+# Superuser, not merely signed in. The names alone say what this instance
 # talks to, and `PUT /{name}` takes any name — so an ordinary account could
 # overwrite the credential a flow authenticates with. `/search` already gates
 # secret names this way and said so; this router was the half that did not.