cli: graceful fallback when --write-env can't write the host-mounted .env

The CLI runs inside the gateway container as the non-root `gateway` user
(uid 10001). The .env file is typically host-mounted at /app/.env, owned
by the host user — so the container process can't write the
.env.tmp file `update_env_file()` creates for the atomic rename. That
surfaced as a raw PermissionError traceback from
`docker exec neuronetz-gateway neuronetz-gateway remove-backend …`.

Now both add-backend and remove-backend catch PermissionError / OSError
on the write, print a tidy red message, and tell the user the two ways
forward: re-run with `docker exec -u root …` (in-container root keeps
the same FS, just bypasses ownership), or paste the printed
OLLAMA_BACKENDS line manually. Exit 1 either way so scripts notice.

src/neuronetz_gateway/cli/manage.py

@@ -448,6 +448,27 @@ def probe_ollama(
     typer.secho("upstream reachable and authenticated.", fg=typer.colors.GREEN, bold=True)
 
 
+def _emit_paste_fallback(path: Path, line: str, exc: BaseException) -> None:
+    """Print a clean fallback message when --write-env can't write the file.
+
+    The bootstrap CLI usually runs inside the gateway container (uid 10001),
+    but the host-mounted .env is typically owned by the host user — so the
+    in-container process can't write it. Instead of crashing with a traceback,
+    print the line the user can paste, and tell them how to retry with the
+    right privileges.
+    """
+    typer.secho(f"✗ could not write {path}: {type(exc).__name__}", fg=typer.colors.RED)
+    typer.echo(f"   ({exc})")
+    typer.echo("")
+    typer.echo("Two ways forward:")
+    typer.echo("  1. Re-run the same command with `docker exec -u root …` so the in-container")
+    typer.echo("     process can write to the host-mounted .env.")
+    typer.echo("  2. Paste this into your .env manually, then `docker compose up -d gateway`:")
+    typer.echo("")
+    typer.echo(f"     OLLAMA_BACKENDS={line}")
+    typer.echo("")
+
+
 @app.command("list-backends")
 def list_backends() -> None:
     """Show the configured Ollama backends (tokens redacted).
@@ -588,7 +609,11 @@ def add_backend(
     line = serialize(updated)
     if write_env is not None:
         path = Path(write_env)
-        update_env_file(path, line)
+        try:
+            update_env_file(path, line)
+        except (PermissionError, OSError) as exc:
+            _emit_paste_fallback(path, line, exc)
+            raise typer.Exit(code=1) from exc
         typer.echo(f"✓ updated {path}")
         typer.echo("   recreate the gateway:  docker compose up -d gateway")
     else:
@@ -643,7 +668,12 @@ def remove_backend(
         return
 
     if write_env is not None:
-        update_env_file(Path(write_env), line)
+        path = Path(write_env)
+        try:
+            update_env_file(path, line)
+        except (PermissionError, OSError) as exc:
+            _emit_paste_fallback(path, line, exc)
+            raise typer.Exit(code=1) from exc
         typer.echo(f"✓ updated {write_env}")
         typer.echo("   recreate the gateway:  docker compose up -d gateway")
     else: