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/flow/dashboards.py

@@ -81,7 +81,7 @@ WidgetType = Literal[
 INPUT_WIDGETS = {"button", "switch", "slider", "input", "dropdown", "color"}
 
 #: What a colour widget puts on the wire, by the format it was configured for.
-#: The default is what the Node-RED installation this ports already sends its
+#: The default is what the Node-RED instance this ports already sends its
 #: DMX encoders — ``[h, s, v]``, hue in degrees and the other two in percent —
 #: and the two alternatives exist because fixtures differ. Mirrored in the
 #: client (``frontend/src/components/Dashboard/ColorWidget.tsx``).
@@ -197,7 +197,7 @@ class WidgetDef(BaseModel):
     differ and a change node per tile is not the answer:
 
     - ``hsv`` (the default) — ``[h, s, v]``, hue 0-360 degrees, saturation and
-      value 0-100 percent. What the Node-RED installation this ports feeds its
+      value 0-100 percent. What the Node-RED instance this ports feeds its
       3CH/4CH DMX encoders, which divide by 360 and by 100.
     - ``rgb`` — ``[r, g, b]``, each 0-255. The conventional range; the
       reference's own encoders produce it after converting.
@@ -479,7 +479,7 @@ class DashboardDef(BaseModel):
     def _flatten(cls, data: Any) -> Any:
         """Read a document written as pages and sections as one grid.
 
-        Stored dashboards live in each installation's git repository, so the
+        Stored dashboards live in each instance's git repository, so the
         old shape is normalised on the way in rather than migrated: an
         untouched document keeps working, and the next save writes it flat.
         """
@@ -502,7 +502,7 @@ class DashboardDef(BaseModel):
         picker records the payload type beside the name, so neither the editor
         nor a wall panel has to fetch the catalogue to know the wiring is
         wrong. A name this build does not know is left alone rather than
-        refused — an older installation reading a newer document simply does
+        refused — an older instance reading a newer document simply does
         not act on it.
         """
         for name, setting in self.settings.items():