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/main.py

@@ -307,7 +307,7 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
             )
         )
         started.append(lambda: run_in_threadpool(close_shared_client))
-        # Optional, and off unless someone enrolled this installation: the
+        # Optional, and off unless someone enrolled this instance: the
         # connector dials the portal, nothing dials in.
         cloud_task: asyncio.Task[None] | None = None
         app.state.cloud_connector = None
@@ -346,7 +346,7 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
             await aclose()
 
 
-# The schema enumerates every endpoint this installation serves, including the
+# The schema enumerates every endpoint this instance serves, including the
 # paths trigger nodes mount at runtime. That is exactly what a developer wants
 # and exactly what an internet-facing deployment should not hand out, so it
 # follows the environment — the same rule the portal's backend uses. The