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>

package-lock.json

@@ -197,16 +197,13 @@
       }
     },
     "apps/PatientDataExtractorService": {
-      "name": "patientdataextractorservice",
-      "hasInstallScript": true
+      "name": "patientdataextractorservice"
     },
     "apps/PaymentOCRService": {
-      "name": "paymentocrservice",
-      "hasInstallScript": true
+      "name": "paymentocrservice"
     },
     "apps/SeleniumService": {
-      "name": "seleniumservice",
-      "hasInstallScript": true
+      "name": "seleniumservice"
     },
     "apps/SeleniumServiceold": {
       "name": "seleniumserviceold",