Add optional Home Assistant integration for battery percentage tracking

MIGRATION.md

@@ -1,3 +1,31 @@
+# Schema Migrations
+
+## Adding Home Assistant fields (ha_entity_id, battery_percentage)
+
+These columns were added in the Home Assistant integration feature. Existing databases
+need a manual migration — `create_all()` does not add columns to existing tables.
+
+**Always snapshot first:**
+```bash
+cp batteries.db batteries.db.$(date +%Y-%m-%d).snapshot
+```
+
+**SQLite:**
+```bash
+sqlite3 batteries.db "ALTER TABLE device ADD COLUMN ha_entity_id VARCHAR(100);"
+sqlite3 batteries.db "ALTER TABLE battery ADD COLUMN battery_percentage INTEGER;"
+```
+
+**MariaDB / MySQL:**
+```sql
+ALTER TABLE device ADD COLUMN ha_entity_id VARCHAR(100) NULL;
+ALTER TABLE battery ADD COLUMN battery_percentage INT NULL;
+```
+
+Both columns are nullable with no default, which is valid on all supported databases.
+
+---
+
 # Migrating from SQLite to MariaDB
 
 This guide covers moving the Battery Tracker database from SQLite to MariaDB/MySQL