feat: embed gf3 water extraction workflow

backend/app/processors/gf3_water/geo.py

@@ -0,0 +1,25 @@
+"""Small geospatial math helpers for lon/lat products."""
+
+from __future__ import annotations
+
+import math
+
+
+def meters_to_degrees(meters: float, lat: float) -> tuple[float, float]:
+    deg_lat = meters / 111_320.0
+    deg_lon = meters / max(111_320.0 * math.cos(math.radians(lat)), 1.0)
+    return deg_lon, deg_lat
+
+
+def pixel_area_m2(info) -> float:
+    center_lat = info.y0 - info.lines * info.dy * 0.5
+    meters_per_deg_lat = 111_320.0
+    meters_per_deg_lon = 111_320.0 * math.cos(math.radians(center_lat))
+    return abs(info.dx * meters_per_deg_lon * info.dy * meters_per_deg_lat)
+
+
+def polygon_area_m2(geom, center_lat: float) -> float:
+    meters_per_deg_lat = 111_320.0
+    meters_per_deg_lon = 111_320.0 * math.cos(math.radians(center_lat))
+    return float(abs(geom.area) * meters_per_deg_lon * meters_per_deg_lat)
+