Adds the second ionospheric data source layer. Polls NOAA SWPC's free
public JSON services every 5 minutes for three products:
* Planetary K-index (1-min cadence) → geomagnetic_observations
* 10.7 cm solar flux (hourly) → solar_flux_observations
* GOES X-ray flux (1-min, 0.1-0.8nm long-wave band only) →
solar_xray_observations
All three products feed HF / Es scoring inputs that the propagation
engine will start using in a follow-up commit:
* Kp drives HF absorption and Es damping (memory notes Kp is inversely
correlated with tropo/Es stability).
* F10.7 is the SSN proxy for ITU-R P.533 HF MUF prediction.
* GOES X-ray long-wave flux is the short-wave fade (SWF) / D-layer
absorption proxy — a C-class flare starts attenuating HF within
seconds of the X-ray peak.
New context `Microwaveprop.SpaceWeather` exposes `upsert_*/1` and
`latest_*/0` per product; `SpaceWeatherFetchWorker` pulls all three
independently so one endpoint's outage doesn't block the others.
Fixtures captured from live SWPC endpoints on 2026-04-15 for the
parser tests.
Not yet wired into scoring — that's a separate commit.
