fix: verbose error handling, batch processing, and multi-file support

- Replace [object Object] errors with readable messages across all 20+ API
  routes by normalizing Zod validation errors to strings (formatZodErrors)
- Add parseApiError() on frontend to defensively handle any details type
- Add global Fastify error handler with full stack traces in logs
- Fix image-to-pdf auth: Object.entries(headers) → headers.forEach()
- Fix passport-photo: safeParse + formatZodErrors, safe error extraction
- Fix OCR silent fallbacks: log exception type/message when falling back,
  include actual engine used in API response and Docker logs
- Fix split tool: process all uploaded images, combine into ZIP with
  subfolders per image
- Fix batch support for blur-faces, strip-metadata, edit-metadata,
  vectorize: add processAllFiles branch for multi-file uploads
- Docker: LOG_LEVEL=debug, PYTHONWARNINGS=default for visibility
- Add Playwright e2e tests verifying all fixes against Docker container

apps/api/src/lib/errors.ts

@@ -0,0 +1,7 @@
+import type { ZodIssue } from "zod";
+
+export function formatZodErrors(issues: ZodIssue[]): string {
+  return issues
+    .map((i) => (i.path.length > 0 ? `${i.path.join(".")}: ${i.message}` : i.message))
+    .join("; ");
+}