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/fluksio/flow/nodes/influx.py

@@ -18,7 +18,7 @@ def _influxdb() -> Any:
     """The client library, which is a `fluksio[server]` extra.
 
     Imported per use rather than at module level, because the node type is
-    registered at boot and an installation with no InfluxDB behind it should
+    registered at boot and an instance with no InfluxDB behind it should
     not have to carry the library to start.
     """
     try:
@@ -198,7 +198,7 @@ class InfluxDbNode(Node):
         queries: dict[str, dict[str, Any]] = {}
 
     @classmethod
-    def instance_key(cls, params: dict[str, Any]) -> str | None:
+    def target_key(cls, params: dict[str, Any]) -> str | None:
         """The bucket, which is the thing several flows share."""
         url, bucket = params.get("url"), params.get("bucket")
         return f"{url}/{bucket}" if url and bucket else None