Remove AddTrailingSlash middleware that broke admin panel

The AddTrailingSlash() Pre middleware was redirecting requests like
/api/admin/users to /api/admin/users/, but admin routes were registered
without trailing slashes, causing routes to not match (404/401 errors).

Mobile API routes already have trailing slashes explicitly defined,
so this middleware was unnecessary and caused conflicts.

Also fix APNS_AUTH_KEY_PATH to use environment variable.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/router/router.go

@@ -50,8 +50,8 @@ func SetupRouter(deps *Dependencies) *echo.Echo {
 	e.Validator = customvalidator.NewCustomValidator()
 	e.HTTPErrorHandler = customHTTPErrorHandler
 
-	// Add trailing slash middleware (before other middleware)
-	e.Pre(middleware.AddTrailingSlash())
+	// NOTE: Removed AddTrailingSlash() middleware - it conflicted with admin routes
+	// which don't use trailing slashes. Mobile API routes explicitly include trailing slashes.
 
 	// Global middleware
 	e.Use(utils.EchoRecovery())