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/conftest.py

@@ -25,7 +25,7 @@ from fluksio.models import (
     RunNode,
     User,
 )
-from tests.utils.portal import INSTALLATION_ID, ISSUER, jwks
+from tests.utils.portal import INSTANCE_ID, ISSUER, jwks
 from tests.utils.user import authentication_token_from_email
 from tests.utils.utils import get_superuser_token_headers
 
@@ -129,7 +129,7 @@ def enrolled(
     portal_key: rsa.RSAPrivateKey,
     db: Session,
 ) -> Any:
-    """Enrol this installation with a fake portal, then undo it."""
+    """Enrol this instance with a fake portal, then undo it."""
     local_user = db.exec(
         select(User).where(User.email == settings.FIRST_SUPERUSER)
     ).one()
@@ -141,8 +141,8 @@ def enrolled(
         cloud_config.CloudConfig(
             portal_url=ISSUER,
             ws_url=f"{ISSUER}/api/v1/tunnel/attach",
-            installation_id=INSTALLATION_ID,
-            token="installation-token",
+            instance_id=INSTANCE_ID,
+            token="instance-token",
             issuer=ISSUER,
             jwks=jwks(portal_key),
             local_user_id=str(local_user.id),
@@ -151,7 +151,7 @@ def enrolled(
         )
     )
     # Enrolment also maps the enrolling account to the portal identity that
-    # owns the installation; without it a portal session resolves to nobody.
+    # owns the instance; without it a portal session resolves to nobody.
     local_user.portal_sub = "portal-user-1"
     db.add(local_user)
     db.commit()