feat: hgb-v3 — Open-Meteo rain features clear the 12h warning gate

The rolling-origin harness (models/eval_rain.json) showed catchment rain
halving flood-year Brier scores, cutting flood-regime MAE 20-40%, and
extending the hard 2024 leads (+6h -> +11h at P.1, +10h -> +19h at
P.103). Ported: train_all loads the catchment-mean series (use_rain /
--no-rain to opt out; without it bundles train as v2), predict fetches
live rain hourly and passes an empty series on failure so rain-trained
bundles serve with NaN features instead of tripping the feature guard,
and the leader worker persists hourly per-point + catchment-mean rows to
a new openmeteo_rain table.

Regenerated backtest: the 2024 record flood now gets a 13-HOUR WARNING
(alert 04:00 vs 17:00 crossing, river at 2.9m at alert time) — the >=12h
acceptance gate PASSES for the first time. Journey on that crossing:
v1 -18h, v2 +6h, v3 +13h. The marginal 2025 double-crest trades its
artifact +46h latch for a calibrated +2h with zero false alarms. P.1
MAE 4.9/7.2/8.7 cm at 6/12/24h. Docs updated throughout.

src/web_api.py

@@ -264,6 +264,27 @@ app.add_middleware(
 )
 
 
+async def _persist_rain():
+    """Save the latest Open-Meteo rain frame into openmeteo_rain (leader only)."""
+    store = app_state.get("forecast_store")  # reuse its SQL engine
+    if not store:
+        return
+    try:
+        from .ml import rain as rain_mod
+
+        def fetch_and_save():
+            frame = rain_mod.fetch_forecast()
+            if not store.engine and not store.connect():
+                return 0
+            return rain_mod.save_to_db(frame, store.engine, store.db_type)
+
+        saved = await asyncio.to_thread(fetch_and_save)
+        if saved:
+            logger.info(f"openmeteo_rain: {saved} hourly rows upserted")
+    except Exception as e:
+        logger.warning(f"rain persistence failed: {e}")
+
+
 async def _precompute_forecasts():
     """Refresh the forecast cache and persist the issued forecasts (leader only)."""
     try:
@@ -351,6 +372,10 @@ async def background_scraping_task():
                         except Exception as e:
                             logger.error(f"HII collection failed: {e}")
 
+                    # Persist the Open-Meteo catchment rain (observed tail +
+                    # 48h forecast) so the DB carries the weather context too
+                    await _persist_rain()
+
                     # Precompute forecasts on fresh data: primes the response
                     # cache (user requests never pay for inference) and records
                     # what the model predicted for later predicted-vs-actual