feat(backend): add web pipeline Phase 1 — XHR interceptor + accessibility snapshot (#1967)

[mrveiss]

Summary

• XHRInterceptor: JS injection to capture fetch/XMLHttpRequest calls with responses
• AccessibilitySnapshot: Playwright accessibility tree capture with text rendering
• find_by_role() and find_by_name() for tree queries (case-insensitive, DFS)
• InterceptedRequest and AccessibilityNode dataclasses with serialization
• 54 tests covering all paths

Closes #1967

Test plan

• JS script generation and idempotency markers
• Result collection: full payloads, errors, evaluate exceptions
• Accessibility capture: None, exceptions, typed attrs, children
• Text rendering: roles, names, values, checked/disabled markers
• Find by role/name: nested, case-insensitive, no match
• Node serialization: optional fields omitted, children recursive
• flake8 passes

🤖 Generated with Claude Code

[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!