feat: openmeteo_rain 2021+ backfill entry point

rain.backfill_db pushes the full cached Open-Meteo archive into the
openmeteo_rain table in 5k-row idempotent upsert chunks;
scripts/backfill_rain_db.py is the thin CLI (DB from Config/.env or
--db-url). Safe to re-run and safe alongside the hourly live writer.

src/ml/rain.py

@@ -162,6 +162,24 @@ def serving_series() -> Optional[pd.Series]:
         return None
 
 
+def backfill_db(engine, db_type: str, chunk_rows: int = 5000) -> int:
+    """Push the full Open-Meteo history (2021+) into openmeteo_rain.
+
+    Loads (or fetches) the archive cache and upserts in chunks; idempotent,
+    safe to re-run, and safe alongside the hourly live writer.
+    """
+    history = load_history()
+    if history is None or history.empty:
+        logger.error("no rain history available to backfill")
+        return 0
+    total = 0
+    for start in range(0, len(history), chunk_rows):
+        part = history.iloc[start: start + chunk_rows]
+        total += save_to_db(part, engine, db_type)
+        logger.info(f"openmeteo_rain backfill: {total}/{len(history)} rows")
+    return total
+
+
 def save_to_db(df: pd.DataFrame, engine, db_type: str) -> int:
     """Upsert per-point + catchment-mean hourly rain into openmeteo_rain.