dashboard: guard stability ratio when every observation is UNKNOWN

[SAY-5]

Problem

Test.stability only treats the "no rolling observations" case specially:

@property
def stability(self) -> float | None:
    if not self.rolling_observations:
        return None

    count_stable   = sum(o.stability is Stability.STABLE   for o in self.rolling_observations)
    count_unstable = sum(o.stability is Stability.UNSTABLE for o in self.rolling_observations)

    return count_stable / (count_stable + count_unstable)

If every rolling observation records Stability.UNKNOWN (the third
variant) — which happens routinely while a test is still being
collected — both counts are 0 and the division raises
ZeroDivisionError. The stack in #245 has it bubbling up through
websocket.send_tests, so every subsequent poll re-triggers the
exception and the dashboard websocket stays broken.

Fix

Compute the denominator explicitly, return None when it is 0, and
keep the existing ratio otherwise.

- return count_stable / (count_stable + count_unstable)
+ total = count_stable + count_unstable
+ if total == 0:
+     return None
+ return count_stable / total

No behaviour change for any test that has at least one stable or
unstable observation — they continue to return the same float as
before.

Fixes #245

[Zac-HD]

Thanks!