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/logic.py

@@ -2,7 +2,7 @@
 
 Anything here could be written as a `python` node — that is what the function
 node is for. These exist because the same handful of shapes account for most of
-a real installation, and a rule you fill in is easier to read on a canvas, and
+a real instance, and a rule you fill in is easier to read on a canvas, and
 to change, than five lines of code repeated eighty times.
 """
 
@@ -115,7 +115,7 @@ class ChangeNode(Node):
     """Reshape a value on its way past: scale, offset, map, or replace.
 
     Node-RED's *change*, which is the second most common node in a real
-    installation after the function.
+    instance after the function.
     """
 
     class Params(BaseModel):