fix: hub-side dedup for ESP32 offline status replay (CUB-239)

- Add migration 002: UNIQUE index on status_logs(camera_id, recorded_at)
- Upgrade migration system to version-tracked (schema_version table)
- Prevents race-condition double-inserts that application-level COUNT(*) check cannot guard against
- Complements existing application-level dedup from CUB-230

internal/db/db.go

@@ -4,6 +4,7 @@ package db
 import (
 	"database/sql"
 	_ "embed"
+	"fmt"
 	"log"
 	"os"
 	"path/filepath"
@@ -14,13 +15,16 @@ import (
 //go:embed migrations/001_create_tables.sql
 var migration001 string
 
+//go:embed migrations/002_dedup_unique_index.sql
+var migration002 string
+
 // DB wraps the sql.DB with connection-level settings.
 type DB struct {
 	*sql.DB
 }
 
 // Open opens the SQLite database at the given path, enables WAL mode,
-// and runs all migrations if the tables don't exist yet.
+// and runs all migrations using a schema_version table for tracking.
 func Open(path string) (*DB, error) {
 	// Ensure the directory exists
 	dir := filepath.Dir(path)
@@ -45,22 +49,46 @@ func Open(path string) (*DB, error) {
 		return nil, err
 	}
 
-	// Check if tables already exist (idempotent migration)
-	var count int
-	if err := db.QueryRow(`
-		SELECT COUNT(*) FROM sqlite_master
-		WHERE type='table' AND name IN ('cameras', 'status_logs', 'recording_events', 'settings')
-	`).Scan(&count); err != nil {
+	// Ensure schema_version table exists for migration tracking
+	if _, err := db.Exec(`CREATE TABLE IF NOT EXISTS schema_version (version INTEGER PRIMARY KEY)`); err != nil {
 		db.Close()
 		return nil, err
 	}
 
-	if count < 4 {
-		log.Printf("Running migrations for %s...", path)
-		if err := migrate(db, migration001); err != nil {
-			db.Close()
-			return nil, err
+	// Read current schema version (0 if table is empty)
+	var currentVersion int
+	if err := db.QueryRow(`SELECT COALESCE(MAX(version), 0) FROM schema_version`).Scan(&currentVersion); err != nil {
+		db.Close()
+		return nil, err
+	}
+
+	// Migration definitions: ordered list of (version, sql)
+	type migration struct {
+		version int
+		sql     string
+	}
+	migrations := []migration{
+		{1, migration001},
+		{2, migration002},
+	}
+
+	for _, m := range migrations {
+		if currentVersion >= m.version {
+			continue
 		}
+		log.Printf("Running migration %d for %s...", m.version, path)
+		if err := migrate(db, m.sql); err != nil {
+			db.Close()
+			return nil, fmt.Errorf("migration %d: %w", m.version, err)
+		}
+		if _, err := db.Exec(`INSERT INTO schema_version (version) VALUES (?)`, m.version); err != nil {
+			db.Close()
+			return nil, fmt.Errorf("record migration %d: %w", m.version, err)
+		}
+		log.Printf("Migration %d complete", m.version)
+	}
+
+	if currentVersion < len(migrations) {
 		log.Println("Migrations complete")
 	}