Add separate power/energy URLs and multipliers for REST smart plugs (#472)

REST/Webhook smart plugs can now fetch power and energy data from
  individual URLs instead of requiring all values in a single status
  response. Each value falls back to the shared Status URL when no
  separate URL is set, preserving backward compatibility. Added power
  and energy multipliers for unit conversion (e.g. 0.001 for Wh→kWh).

backend/app/core/database.py

@@ -1610,6 +1610,24 @@ async def run_migrations(conn):
     except OperationalError:
         pass  # Already applied
 
+    # Migration: Add separate REST power/energy URLs and multipliers
+    try:
+        await conn.execute(text("ALTER TABLE smart_plugs ADD COLUMN rest_power_url VARCHAR(500)"))
+    except OperationalError:
+        pass  # Already applied
+    try:
+        await conn.execute(text("ALTER TABLE smart_plugs ADD COLUMN rest_power_multiplier REAL DEFAULT 1.0"))
+    except OperationalError:
+        pass  # Already applied
+    try:
+        await conn.execute(text("ALTER TABLE smart_plugs ADD COLUMN rest_energy_url VARCHAR(500)"))
+    except OperationalError:
+        pass  # Already applied
+    try:
+        await conn.execute(text("ALTER TABLE smart_plugs ADD COLUMN rest_energy_multiplier REAL DEFAULT 1.0"))
+    except OperationalError:
+        pass  # Already applied
+
     # Migration: Add batch_id column to print_queue for batch grouping
     try:
         await conn.execute(