fix: preserve provider operation failures (#163)

[atomantic]

Summary

• distinguish verified logged-out and empty-tree states from browser or provider scraper failures
• return structured HTTP 503 errors with provider and operation context, including partial aggregate-session results
• keep provider-session failures visible in the UI with browser-settings and retry remediation

Tests

• npm test -- --run tests/unit/services/providerService.spec.ts tests/integration/api/providers.spec.ts (11 passed)
• npm test -- --run (186 passed, 2 skipped)
• npm run build
• npm run check:file-sizes

Review

• Claude local review at medium effort completed for the configured one-round cap
• addressed both findings: aggregate result preservation and aggregate failure coverage

Closes #163