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/tests/flow/test_webpush.py

@@ -16,7 +16,7 @@ from fluksio.flow import webpush
 
 @pytest.fixture(autouse=True)
 def store(tmp_path, monkeypatch):
-    """A store per test, so nothing writes into the real installation."""
+    """A store per test, so nothing writes into the real instance."""
     monkeypatch.setattr(settings, "WEBPUSH_FILE", tmp_path / "webpush.json")
     return tmp_path / "webpush.json"