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/placement.py

@@ -10,7 +10,7 @@ counted at all.
 One question, then, asked once here: of every machine attached — this one and
 each worker — which could grant what this node asked for, and which of those
 has it free right now. A worker reports its inventory when it attaches, so the
-answer covers the whole installation rather than the host the engine happens to
+answer covers the whole instance rather than the host the engine happens to
 be on.
 
 Two properties are worth stating because they are what make the waiting safe:
@@ -178,7 +178,7 @@ class Placer:
         """Every machine's size, as something to measure a request against.
 
         A provisioner's job shapes count: a machine it can start on demand is
-        one this installation has, even when nothing is attached yet.
+        one this instance has, even when nothing is attached yet.
         """
         shapes = []
         for target in self.targets():