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)

pkg/models/camera.go

@@ -7,11 +7,12 @@ import (
 
 // Camera represents a registered GoPro camera in the system.
 type Camera struct {
-	CameraID      string    `json:"camera_id"`
-	FriendlyName  string    `json:"friendly_name"`
-	MacAddress    string    `json:"mac_address,omitempty"`
-	CreatedAt     time.Time `json:"created_at"`
-	UpdatedAt     time.Time `json:"updated_at"`
+	CameraID                  string    `json:"camera_id"`
+	FriendlyName              string    `json:"friendly_name"`
+	MacAddress                string    `json:"mac_address,omitempty"`
+	BatteryCalibrationOffset  *float64  `json:"battery_calibration_offset,omitempty"`
+	CreatedAt                 time.Time `json:"created_at"`
+	UpdatedAt                 time.Time `json:"updated_at"`
 }
 
 // StatusLog records a single status poll from an ESP8266 node.