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/api/routes/test_flavors.py

@@ -9,7 +9,7 @@ from fluksio.core.db import seed_flavors
 from fluksio.models import Flavor
 
 
-def test_an_installation_starts_with_sizes_to_pick_from(
+def test_an_instance_starts_with_sizes_to_pick_from(
     client: TestClient, superuser_token_headers: dict[str, str]
 ) -> None:
     response = client.get(
@@ -20,7 +20,7 @@ def test_an_installation_starts_with_sizes_to_pick_from(
     assert {"small", "medium", "large", "gpu-small"} <= names
 
 
-def test_seeding_leaves_an_installation_that_has_its_own_alone(db: Session) -> None:
+def test_seeding_leaves_an_instance_that_has_its_own_alone(db: Session) -> None:
     """Re-adding a size somebody deliberately removed is an argument to avoid."""
     before = {row.name for row in db.exec(select(Flavor)).all()}
     seed_flavors(db)