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.

backend/fluksio/core/bootstrap.py

@@ -1,6 +1,6 @@
 """First run: an account to sign in with.
 
-An installation started from the command line is given no environment, so the
+An instance started from the command line is given no environment, so the
 superuser a deployment sets in `.env` has to come from somewhere. The session
 key is the CLI's own business — it has to be set before this module can be
 imported at all.
@@ -27,7 +27,7 @@ def ensure_superuser(
     """The account to sign in with, made on first run.
 
     Returns the user and, when it was just created, the password in clear —
-    the caller prints it once. An installation that already has a superuser is
+    the caller prints it once. An instance that already has a superuser is
     left alone: this is a first run, not a password reset.
     """
     existing = session.exec(
@@ -57,7 +57,7 @@ def pick_superuser(session: Session, email: str | None = None) -> User:
         select(User).where(User.is_superuser == True)  # noqa: E712
     ).all()
     if not users:
-        raise LookupError("This installation has no superuser to enrol as")
+        raise LookupError("This instance has no superuser to enrol as")
     if len(users) > 1:
         addresses = ", ".join(sorted(u.email for u in users))
         raise LookupError(f"Several superusers here — name one with --as: {addresses}")