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/fluksio/main.py

@@ -44,7 +44,8 @@ def custom_generate_unique_id(route: APIRoute) -> str:
 
 
 if settings.SENTRY_DSN and settings.ENVIRONMENT != "local":
-    sentry_sdk.init(dsn=str(settings.SENTRY_DSN), enable_tracing=True)
+    # `enable_tracing` was removed in sentry-sdk 2.x; this is what it meant.
+    sentry_sdk.init(dsn=str(settings.SENTRY_DSN), traces_sample_rate=1.0)
 
 
 def _state_backend() -> StateBackend: