Compare: resolve a location's name immediately on add

Add a lightweight GET /api/v2/place?lat=&lon= endpoint that snaps to the grid
cell and returns the reverse-geocoded "neighborhood, city, region" label (the
same string the view endpoints expose as place, cached + throttled). On the
compare page, adding a location now fires this right away so the chip flips from
coordinates to the place name immediately — instead of only resolving when the
full comparison loads on Refresh. Best-effort: the series load still resolves the
name, so a failed/again call is harmless.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B3Q2EkHHnTUX2BfhV5q1Zc

app.py

@@ -307,6 +307,25 @@ def api_geocode(q: str = Query(..., min_length=1)):
         raise HTTPException(status_code=502, detail=f"geocoding failed: {e}")
 
 
+def api_place(
+    lat: float = Query(..., ge=-90, le=90),
+    lon: float = Query(..., ge=-180, le=180),
+):
+    """Best-effort neighbourhood/city label for a point — the same string the view
+    endpoints expose as ``place`` (snapped to the grid cell, reverse-geocoded and
+    cached). Resolved on its own so the compare page can show a location's name as
+    soon as it's added, before its full series loads."""
+    if not grid.in_north_america(lat, lon):
+        return {"place": None}
+    cell = grid.snap(lat, lon)
+    with audit.RunAudit(endpoint="place", lat=round(lat, 4), lon=round(lon, 4),
+                        cell_id=cell["id"]) as run:
+        with run.phase("reverse_geocode"):
+            place = climate.reverse_geocode(cell["center_lat"], cell["center_lon"])
+    return {"place": place,
+            "cell": {"center_lat": cell["center_lat"], "center_lon": cell["center_lon"]}}
+
+
 def api_grade(
     request: Request,
     lat: float = Query(..., ge=-90, le=90),
@@ -669,6 +688,7 @@ v1.add_api_route("/grade", api_grade, methods=["GET"])
 
 v2 = APIRouter(tags=["v2"])
 v2.add_api_route("/geocode", api_geocode, methods=["GET"])
+v2.add_api_route("/place", api_place, methods=["GET"])
 v2.add_api_route("/grade", api_grade, methods=["GET"])
 v2.add_api_route("/calendar", api_calendar, methods=["GET"])
 v2.add_api_route("/day", api_day, methods=["GET"])