Track battery percentage history; skip poll write when value unchanged

MIGRATION.md

@@ -1,5 +1,42 @@
 # Schema Migrations
 
+## Adding battery_pct_log table
+
+This table was added to track battery percentage change history.
+
+**Always snapshot first:**
+```bash
+cp batteries.db batteries.db.$(date +%Y-%m-%d).snapshot
+```
+
+**SQLite (Python):**
+```python
+import sqlite3
+conn = sqlite3.connect('batteries.db')
+conn.execute('''CREATE TABLE IF NOT EXISTS battery_pct_log (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  battery_id INTEGER NOT NULL REFERENCES battery(id) ON DELETE CASCADE,
+  percentage INTEGER NOT NULL,
+  recorded_at VARCHAR(19) NOT NULL,
+  source VARCHAR(10)
+)''')
+conn.commit(); conn.close()
+```
+
+**MariaDB / MySQL:**
+```sql
+CREATE TABLE IF NOT EXISTS battery_pct_log (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  battery_id INT NOT NULL,
+  percentage INT NOT NULL,
+  recorded_at VARCHAR(19) NOT NULL,
+  source VARCHAR(10),
+  CONSTRAINT fk_bpl_battery FOREIGN KEY (battery_id) REFERENCES battery(id) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+```
+
+---
+
 ## Adding Home Assistant fields (ha_entity_id, battery_percentage)
 
 These columns were added in the Home Assistant integration feature. Existing databases