One installation per project, and no login to reach it

Two things a local install should not have asked for.

`fluksio serve` now signs you in. Logging in to your own machine was a
formality — the password was printed by the same process that would have
checked it, and the database it authenticates against sits in the directory
the token goes into — so `serve` mints the token itself and says where it put
it. `fluksio login` is left for an engine somewhere else.

And an installation is `.fluksio` beside the code, found the way `.git` is,
rather than one `~/.fluksio` for the machine. A repository with its own venv
was already getting its own engine; it now gets its own flows, run history and
token too, instead of three repositories sharing one database and fighting
over one port. `--global` asks for the shared one, `--data-dir` still names
any directory, and when both exist the banner says which you are looking at
and how to reach the other.

The directory ignores itself from within — a `.gitignore` of `*`, the way uv
writes one into `.venv` — because it holds a credential and a database, and
neither belongs in anybody's history. The token is written mode 600. A login
an older version wrote to ~/.config/fluksio is still read, so nothing that
worked stops working.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ue1tkFWB1bcGy3aWhCKpU

backend/tests/test_cli.py

@@ -86,3 +86,33 @@ def test_run_arguments_are_typed_by_the_flow_they_are_for() -> None:
 
     with pytest.raises(SyncError, match="not an input of this flow"):
         _params(definition, ["--nonesuch", "1"])
+
+
+def test_serve_uses_the_installation_the_directory_belongs_to(
+    tmp_path: Path, monkeypatch
+) -> None:
+    """A repository with its own venv gets its own engine, not the machine's."""
+    from fluksio import cli
+
+    monkeypatch.chdir(tmp_path)
+    monkeypatch.setattr(cli, "DEFAULT_HOME", tmp_path / "home" / ".fluksio")
+
+    # Nothing above it yet: one is made here rather than in the home directory.
+    made = cli._data_dir(None)
+    assert made == (tmp_path / ".fluksio").resolve()
+    assert (made / ".gitignore").exists()
+
+    # And is then found again from anywhere below it.
+    deep = tmp_path / "src" / "deep"
+    deep.mkdir(parents=True)
+    monkeypatch.chdir(deep)
+    assert cli._data_dir(None) == made
+
+    # `--global` asks for the shared one even so.
+    assert (
+        cli._data_dir(None, shared=True) == (tmp_path / "home" / ".fluksio").resolve()
+    )
+
+    # And `--data-dir` still names any directory outright.
+    named = cli._data_dir(str(tmp_path / "named"))
+    assert named == (tmp_path / "named").resolve()