refactor(ocr): remove subprocess mode — HTTP only

- ocr.js: Removed all subprocess/spawn logic (spawn, fs, path, process
  imports, resolveWorkerCommand, buildPythonPath, buildWorkerError,
  callOcrViaSubprocess, resolveOcrMode, runOcrCommand, etc.)
  HTTP is now the only mode. Added validation when OCR_BASE_URL is not
  configured. ~281 lines → ~80 lines.
- default.cjs: Removed projectRoot from ocr config (kept path import
  for data.root)
- runtime.js: Removed OCR_PROJECT_ROOT env var parsing
- runtime-config.js: Removed projectRoot from ocr type definition

All 5 exports preserved: recognizeTencentCaptcha,
recognizeImageCaptcha, batchRecognizeTencentCaptcha,
warmupLocalOcrWorker, closeLocalOcrWorker

apps/backend/src/config/runtime.js

@@ -128,11 +128,6 @@ function applyEnvOverrides(baseConfig) {
   if (sessionDebug !== null) {
     nextConfig.session.debug = sessionDebug;
   }
-  const ocrProjectRoot = String(process.env.OCR_PROJECT_ROOT || "").trim();
-  if (ocrProjectRoot) {
-    nextConfig.ocr.projectRoot = ocrProjectRoot;
-  }
-
   const ocrBaseUrl = String(process.env.OCR_BASE_URL || "").trim();
   if (ocrBaseUrl) {
     nextConfig.ocr.baseUrl = ocrBaseUrl;