fix: decouple Python venv setup from npm install postinstall

The three Python services (PatientDataExtractorService, PaymentOCRService,
SeleniumService) provisioned their venvs via postinstall, chained into
root's own postinstall. A failure in any one of them aborted the whole
npm install, which could leave unrelated Node deps (e.g. Backend's sharp)
uninstalled — silently breaking the app with no indication why.

Renamed each service's postinstall to an explicit "setup" script, added a
root "npm run setup:python" to run all three, and documented it as its
own step in the README instead of an automatic install-time side effect.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/PatientDataExtractorService/package.json

@@ -2,7 +2,7 @@
   "name": "patientdataextractorservice",
   "private": true,
   "scripts": {
-    "postinstall": "python3 -m venv .venv && .venv/bin/pip install -r requirements.txt",
+    "setup": "python3 -m venv .venv && .venv/bin/pip install -r requirements.txt",
     "dev": ".venv/bin/python3 main.py"
   }
 }