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/config.py

@@ -54,7 +54,7 @@ class Settings(BaseSettings):
     FRONTEND_HOST: str = "http://localhost:5173"
     ENVIRONMENT: Literal["local", "staging", "production"] = "local"
 
-    #: Everything this installation keeps: the database, the flow repository,
+    #: Everything this instance keeps: the database, the flow repository,
     #: secrets, artifacts and the user venv. The paths below derive from it
     #: unless they are set explicitly.
     DATA_DIR: Path = Path("flow-data")
@@ -72,13 +72,13 @@ class Settings(BaseSettings):
     # Which failures reach which channel. Beside the flows, not in them:
     # alerting is the deployment's concern, not any one flow's.
     ALERTS_FILE: Path = Path("flow-data/alerts.json")
-    # This installation's web push keypair and the browsers subscribed to it.
+    # This instance's web push keypair and the browsers subscribed to it.
     # Beside the alerts it serves; deleting it makes every device subscribe
     # again.
     WEBPUSH_FILE: Path = Path("flow-data/webpush.json")
     # Where machines can be started from when a node needs one and nothing that
     # could take it is attached. Operator-authored, like the alerts beside it,
-    # and absent on an installation that has nowhere to start one.
+    # and absent on an instance that has nowhere to start one.
     PROVISIONERS_FILE: Path = Path("flow-data/provisioners.json")
     # Which dashboards each device shows. Beside the flows for the same reason
     # alerting is: where a screen hangs is the deployment's concern rather than
@@ -99,7 +99,7 @@ class Settings(BaseSettings):
     PRIVATE_API_ENABLED: bool = False
     DOMAIN: str = "localhost"
     OAUTH_PRIVATE_KEY_FILE: Path = Path("flow-data/oauth-key.pem")
-    # Written only when someone enrols this installation with a portal.
+    # Written only when someone enrols this instance with a portal.
     # Its absence is what keeps remote access off.
     CLOUD_CONFIG_FILE: Path = Path("flow-data/cloud.json")
     OAUTH_CODE_EXPIRE_SECONDS: int = 60
@@ -113,7 +113,7 @@ class Settings(BaseSettings):
     # because a limit somebody set and did not get is the worse surprise.
     FLOW_MAX_WORKERS: PositiveInt = 4
     # How many cascades may be in flight at once. Sustained throughput is this
-    # over the mean cascade time, so an installation whose nodes wait on the
+    # over the mean cascade time, so an instance whose nodes wait on the
     # network rather than on a CPU wants it higher than the core count.
     FLOW_MAX_CASCADES: PositiveInt = 4
     # How many batch runs are driven at once. A different limit from the one