Feature: Abrechnungsperiode + Strompreise im Energie-Dashboard (v1.7.0)

- history.py: Tabelle period_starts — speichert kWh-Zählerstand zu Monats-/Jahresbeginn
- main.py: price_import/price_export in Config; /api/period-energy Endpoint
- Web UI: Preisfelder in Einstellungen (€/kWh Bezug + Vergütung)
- Energie-Dashboard: Cards zeigen Monat/Jahr kWh + Kosten statt All-Time-Total

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

haos-addon/src/history.py

@@ -2,7 +2,7 @@ import logging
 import sqlite3
 import threading
 import time
-from typing import Dict, List, Tuple
+from typing import Dict, List, Optional, Tuple
 
 log = logging.getLogger(__name__)
 
@@ -37,11 +37,51 @@ def init_db():
             CREATE INDEX IF NOT EXISTS idx_inv_sensor_ts
             ON measurements(inv_id, sensor_id, ts)
         """)
+        # Periodenstarts: kWh-Zählerstand zu Beginn jeder Abrechungsperiode
+        c.execute("""
+            CREATE TABLE IF NOT EXISTS period_starts (
+                agg_id      TEXT NOT NULL,
+                period_type TEXT NOT NULL,
+                period_key  TEXT NOT NULL,
+                value       REAL NOT NULL,
+                PRIMARY KEY (agg_id, period_type, period_key)
+            )
+        """)
         c.commit()
     cleanup_old()
     log.info("History DB initialisiert: %s", DB_PATH)
 
 
+def period_key(period_type: str) -> str:
+    import datetime
+    now = datetime.date.today()
+    return now.strftime("%Y-%m") if period_type == "monthly" else now.strftime("%Y")
+
+
+def save_period_start_if_new(agg_id: str, period_type: str, key: str, current_value: float):
+    """Speichert den Startwert nur wenn diese Periode noch nicht existiert."""
+    with _lock:
+        c = _get_conn()
+        c.execute("""
+            INSERT OR IGNORE INTO period_starts(agg_id, period_type, period_key, value)
+            VALUES (?, ?, ?, ?)
+        """, (agg_id, period_type, key, current_value))
+        c.commit()
+
+
+def get_period_consumption(agg_id: str, period_type: str, key: str,
+                           current_value: float) -> Optional[float]:
+    """Verbrauch seit Periodenbeginn; None wenn noch kein Startwert gespeichert."""
+    with _lock:
+        row = _get_conn().execute("""
+            SELECT value FROM period_starts
+            WHERE agg_id=? AND period_type=? AND period_key=?
+        """, (agg_id, period_type, key)).fetchone()
+    if row is None:
+        return None
+    return max(0.0, current_value - row[0])
+
+
 def write_batch(inv_id: str, ts: float, values: Dict[str, float]):
     rows = [(inv_id, sid, ts, val) for sid, val in values.items()]
     with _lock: