refactor(e2e): replace HttpLibrary with RequestsLibrary

- Swap HttpLibrary for RequestsLibrary in common.resource
- Add robotframework-requests to requirements
- Remove backend health check from suite setup (setup moved to individual tests)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

e2e/resources/common.resource

@@ -1,12 +1,9 @@
 *** Settings ***
 Library    Browser
-Library    HttpLibrary
+Library    RequestsLibrary
 
 Variables    ${CURDIR}/../../.env
 
-# Health check timeout for suite setup (120 s poll interval).
-Suite Setup    Wait For Backend Health    timeout=120    interval=5
-
 *** Variables ***
 ${FRONTEND_URL}    http://localhost:5173
 ${BACKEND_URL}     http://localhost:8000