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_panels.py

@@ -544,7 +544,7 @@ def test_removing_the_panel_revokes_its_credential(
 
 
 # --------------------------------------------------------------------------
-# A screen that reached the portal but not this installation
+# A screen that reached the portal but not this instance
 # --------------------------------------------------------------------------
 
 
@@ -592,7 +592,7 @@ def test_a_remote_device_is_paired_at_the_portal(
 ) -> None:
     """A device that arrived through the tunnel gets the portal's credential.
 
-    It could never present one this installation signed: the portal verifies
+    It could never present one this instance signed: the portal verifies
     what crosses it, and it verifies against its own key.
     """
     import httpx
@@ -627,7 +627,7 @@ def test_a_remote_device_is_paired_at_the_portal(
     assert approved.status_code == 200, approved.text
 
     assert calls[0]["url"].endswith("/api/v1/panel-tokens/")  # type: ignore[union-attr]
-    assert calls[0]["headers"]["Authorization"] == "Bearer installation-token"  # type: ignore[index]
+    assert calls[0]["headers"]["Authorization"] == "Bearer instance-token"  # type: ignore[index]
     assert calls[0]["json"] == {"panel": "hallway"}  # type: ignore[index]
 
     collected = client.get(