Add the fluksio CLI: serve, enroll, worker

`pip install fluksio && fluksio serve` on a machine with no Docker, no
database and no configuration — which is the case this is for: a node on
a cluster where ports cannot be opened. It makes its data directory, its
key and an admin account, prints the password once, and serves. Pairing
is `fluksio enroll <code> --portal …`, doing what the Settings screen
does through the same function, before the engine starts and without one
running — a machine nobody can route to has no browser pointed at it
either. The portal serves the dashboard, so nothing is served here.

Two things had to give way. `fastapi[standard]` pulls a cloud CLI that
wants sentry-sdk 2.x while we pinned below it — no pip resolution
existed, so the pin is lifted, which the comment beside it had been
waiting for and which also lets the Python cap go. And `uv` is now a
dependency rather than something to find on PATH: the Modules screen is
how a data scientist installs torch, and it was quietly falling back to
the engine's own interpreter.

The CLI imports nothing from the engine before it has set DATA_DIR — the
settings are built on the first import of core.config, and reaching it
early put the database in the working directory. There is a test for
that now, because the failure is silent.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/test_cli.py

@@ -0,0 +1,36 @@
+"""The command line, and the one ordering it depends on."""
+
+import subprocess
+import sys
+from pathlib import Path
+
+from fluksio.cli import load_or_create_secret_key
+
+
+def test_importing_the_cli_does_not_build_the_settings() -> None:
+    """`_configure_environment` has to run before anything reads a setting.
+
+    The settings are built on the first import of `fluksio.core.config`, and
+    every engine module reaches it within an import or two. If importing the
+    CLI pulled it in, `DATA_DIR` would be fixed at whatever directory the
+    command was run from — which is how the database ends up in the cwd.
+    """
+    leaked = subprocess.run(
+        [
+            sys.executable,
+            "-c",
+            "import fluksio.cli, sys; print('fluksio.core.config' in sys.modules)",
+        ],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    assert leaked.stdout.strip() == "False", leaked.stdout
+
+
+def test_the_secret_key_is_kept_rather_than_regenerated(tmp_path: Path) -> None:
+    """A new key each start would sign out every session and orphan secrets.enc."""
+    path = tmp_path / "secret_key"
+    first = load_or_create_secret_key(path)
+    assert load_or_create_secret_key(path) == first
+    assert path.stat().st_mode & 0o777 == 0o600