fix(backend): bootstrap admin from env instead of seeding admin/admin

007_seed_data.sql shipped a fixed admin account whose bcrypt hash decodes
to the password "admin", giving every deployment the same known
credentials. The seed row is removed; UserService.EnsureAdmin now creates
the administrator on startup from ADMIN_USERNAME / ADMIN_PASSWORD. It is
idempotent and never overwrites an existing password, so an operator who
rotates the admin password keeps it across restarts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/config/config.go

@@ -18,6 +18,10 @@ type Config struct {
 	JWTAccessTTL  time.Duration
 	JWTRefreshTTL time.Duration
 
+	// Initial admin bootstrap (applied on startup if the user does not exist)
+	AdminUsername string
+	AdminPassword string
+
 	// Database
 	DatabaseURL string
 
@@ -87,6 +91,9 @@ func Load() (*Config, error) {
 		JWTAccessTTL:  parseDuration("JWT_ACCESS_TTL", "15m"),
 		JWTRefreshTTL: parseDuration("JWT_REFRESH_TTL", "720h"),
 
+		AdminUsername: defaultStr("ADMIN_USERNAME", "admin"),
+		AdminPassword: requireStr("ADMIN_PASSWORD"),
+
 		DatabaseURL: requireStr("DATABASE_URL"),
 
 		FilesPath:       requireStr("FILES_PATH"),