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/core/db.py

@@ -142,9 +142,9 @@ def init_db(session: Session) -> None:
         user = crud.create_user(session=session, user_create=user_in)
 
 
-#: The sizes an installation starts with. Written once, when there are none,
+#: The sizes an instance starts with. Written once, when there are none,
 #: and editable from there — what a name means is a property of the machines
-#: this installation has, and nothing here knows what those are.
+#: this instance has, and nothing here knows what those are.
 SEED_FLAVORS: tuple[dict[str, Any], ...] = (
     {"name": "small", "cpus": 1, "ram": 2048, "description": "A poll, a threshold"},
     {"name": "medium", "cpus": 4, "ram": 8192, "description": "A step that computes"},
@@ -160,7 +160,7 @@ SEED_FLAVORS: tuple[dict[str, Any], ...] = (
 
 
 def seed_flavors(session: Session) -> None:
-    """Give a new installation sizes to pick from, once.
+    """Give a new instance sizes to pick from, once.
 
     Only when there are none at all: they are editable, and re-adding one that
     somebody deliberately removed would be an argument nobody can win.