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/migrations/001_create_tables.sql

@@ -3,11 +3,12 @@
 
 -- Cameras table: registry of all GoPro cameras
 CREATE TABLE IF NOT EXISTS cameras (
-    camera_id     TEXT PRIMARY KEY,
-    friendly_name TEXT NOT NULL,
-    mac_address   TEXT UNIQUE,
-    created_at    DATETIME NOT NULL DEFAULT (datetime('now')),
-    updated_at    DATETIME NOT NULL DEFAULT (datetime('now'))
+    camera_id                    TEXT PRIMARY KEY,
+    friendly_name                TEXT NOT NULL,
+    mac_address                  TEXT UNIQUE,
+    battery_calibration_offset   REAL,
+    created_at                   DATETIME NOT NULL DEFAULT (datetime('now')),
+    updated_at                   DATETIME NOT NULL DEFAULT (datetime('now'))
 );
 
 CREATE INDEX IF NOT EXISTS idx_cameras_mac ON cameras(mac_address);

internal/db/migrations/002_add_camera_calibration.sql

@@ -0,0 +1,5 @@
+-- Migration 002: Add battery_calibration_offset to cameras table
+-- This column stores a per-camera calibration value for converting raw battery
+-- readings (e.g. GoPro Hero 3 byte at offset 57) into percentage values.
+
+ALTER TABLE cameras ADD COLUMN battery_calibration_offset REAL;