CUB-228: add battery_calibration_offset to cameras table

- Add column to 001_create_tables.sql for fresh databases
- Add migration 002 for existing databases (idempotent via
  pragma_table_info check)
- Implement runIncrementalMigrations in db.go
- Add BatteryCalibrationOffset to Camera model
- Update all camera SELECT queries (cameras List, detail, MQTT
  subscriber getCamera, register)

internal/db/db.go

@@ -14,6 +14,9 @@ import (
 //go:embed migrations/001_create_tables.sql
 var migration001 string
 
+//go:embed migrations/002_add_camera_calibration.sql
+var migration002 string
+
 // DB wraps the sql.DB with connection-level settings.
 type DB struct {
 	*sql.DB
@@ -62,6 +65,12 @@ func Open(path string) (*DB, error) {
 			return nil, err
 		}
 		log.Println("Migrations complete")
+	} else {
+		// Run incremental migrations on existing databases
+		if err := runIncrementalMigrations(db); err != nil {
+			db.Close()
+			return nil, err
+		}
 	}
 
 	return &DB{db}, nil
@@ -83,6 +92,25 @@ func migrate(db *sql.DB, sql string) error {
 	return nil
 }
 
+// runIncrementalMigrations applies migrations that haven't been run yet on
+// an existing database (one where the 001 schema already exists).
+func runIncrementalMigrations(db *sql.DB) error {
+	// Migration 002: add battery_calibration_offset if it doesn't exist
+	var colCount int
+	err := db.QueryRow(`SELECT COUNT(*) FROM pragma_table_info('cameras') WHERE name = 'battery_calibration_offset'`).Scan(&colCount)
+	if err != nil {
+		return err
+	}
+	if colCount == 0 {
+		log.Println("Running migration 002: add battery_calibration_offset")
+		if err := migrate(db, migration002); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
 // splitSQL splits a SQL string on semicolons, respecting quoted strings.
 func splitSQL(sql string) []string {
 	var stmts []string