fix: use MAX_AOI_KM2 from config instead of hardcoded 10,000 km² in models

Backend now enforces the same 500 km² limit shown in config.py.
Previously models.py had its own MAX_AREA_KM2 = 10,000 constant
that was never connected to the configurable value.

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via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

app/models.py

@@ -10,7 +10,6 @@ from pyproj import Geod
 
 # --- East Africa bounding box (approximate) ---
 EA_BOUNDS = (22.0, -5.0, 52.0, 23.0)  # (min_lon, min_lat, max_lon, max_lat)
-MAX_AREA_KM2 = 10_000
 MAX_LOOKBACK_DAYS = 3 * 365 + 1  # ~3 years
 
 
@@ -55,9 +54,10 @@ class AOI(BaseModel):
         min_lon, min_lat, max_lon, max_lat = self.bbox
         ea_min_lon, ea_min_lat, ea_max_lon, ea_max_lat = EA_BOUNDS
         # Check area first so "too large" error takes priority
-        if self.area_km2 > MAX_AREA_KM2:
+        from app.config import MAX_AOI_KM2
+        if self.area_km2 > MAX_AOI_KM2:
             raise ValueError(
-                f"AOI area ({self.area_km2:.0f} km²) exceeds 10,000 km² limit"
+                f"AOI area ({self.area_km2:.0f} km²) exceeds {MAX_AOI_KM2:,} km² limit"
             )
         if (
             max_lon < ea_min_lon