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/alembic/versions/c7e2b9f34a15_flavor_table.py

@@ -2,7 +2,7 @@
 
 A named amount of machine — cores, cards and memory — so a node can ask for one
 by name. Seeded with a few sizes on first start, and editable afterwards: what
-"gpu-small" means is a property of the machines an installation has, and those
+"gpu-small" means is a property of the machines an instance has, and those
 change.
 
 Revision ID: c7e2b9f34a15