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/models.py

@@ -53,9 +53,9 @@ class User(UserBase, table=True):
     id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True)
     hashed_password: str
     #: The portal account this local user stands for, if any. Set for the
-    #: superuser who enrolled this installation and for every remote user one
+    #: superuser who enrolled this instance and for every remote user one
     #: of them admitted; None for a purely local account, which is what an
-    #: installation nobody enrolled has only.
+    #: instance nobody enrolled has only.
     portal_sub: str | None = Field(default=None, max_length=64, unique=True, index=True)
     created_at: datetime | None = Field(
         default_factory=get_datetime_utc,
@@ -491,7 +491,7 @@ class FlavorBase(SQLModel):
     """A named amount of machine, so a node can ask for one by name.
 
     The point is not to save typing. Cores and megabytes are a property of the
-    machines an installation actually has, and they change when those machines
+    machines an instance actually has, and they change when those machines
     do — so a node saying "gpu-small" keeps meaning something after the cluster
     is replaced, where a node saying 8 and 16384 quietly stops.
     """