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/utils/portal.py

@@ -14,7 +14,7 @@ import jwt
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import rsa
 
-INSTALLATION_ID = "6f1c2d3e-4a5b-4c6d-8e9f-0a1b2c3d4e5f"
+INSTANCE_ID = "6f1c2d3e-4a5b-4c6d-8e9f-0a1b2c3d4e5f"
 ISSUER = "https://hub.example.test"
 
 
@@ -28,7 +28,7 @@ def portal_token(
     key: rsa.RSAPrivateKey,
     *,
     subject: str = "portal-user-1",
-    audience: str = INSTALLATION_ID,
+    audience: str = INSTANCE_ID,
     issuer: str = ISSUER,
     scope: str = "proxy",
 ) -> str: