GET /api/satellites?groups=stations,weather returns last-known satellite
positions propagated from CelesTrak GP JSON (OMM mean elements, not TLE)
via the real sgp4 library (TEME->geodetic), not two-body Kepler.
- Fetch gp.php?GROUP=...&FORMAT=JSON at most once per 2h per group,
cached with a last-good blob fallback for 403 / stale responses.
- SatNOGS TLE fallback only when the CelesTrak cache is empty.
- bbox viewport culling; unknown group -> 400.
- overlay_catalog() gains id=satellites (kind=points, /api/satellites).
- OMM path handles NORAD cat numbers >= 100000 (no TLE round-trip).
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