refactor: notification metrics come from spans, and NotificationMetrics is gone

[HardMax71]

Summary

Part of #389, the notifications area. The ten instruments in NotificationMetrics now come from spans, so NotificationService has no meter left.

Changes

Each delivery runs in a notification.deliver span with category, channel, severity, result, error type and delivery seconds. The throttle, each retry and each subscription update leave a one-line notification.throttled, notification.retry or notification.subscription_change span where the metric call was. The collector's signaltometrics connector makes the same ten series from them. Labels take the span attribute names (notification_channel, notification_error, ...), so the notifications dashboard's queries and legends move with them. NotificationService takes trace.Tracer.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or tooling changes

Test Plan

• Backend unit tests pass (cd backend && uv run pytest tests/unit)
• Frontend unit tests pass (cd frontend && npm run test)
• Backend E2E tests pass (docker compose exec -T backend sh -c 'uv sync --group test --no-dev --frozen --no-install-project --no-build && pytest tests/e2e')
• Frontend E2E tests pass (cd frontend && npm run test:e2e)
• API spec and client regenerated if routes or schemas changed (./deploy.sh openapi, then ./deploy.sh types), never edited by hand
• Manual testing performed

Validation: unit 570 passed (6 span tests), contract 3 passed, e2e 488 passed and 1 skipped. Driving NotificationService from the API container against a local stack (3 in-app deliveries, 1 webhook to a closed port, 2 subscription updates) gave exactly those counts in VictoriaMetrics, with 2 retries and ConnectError on the failure. The throttle path is covered by a unit test only.

Checklist

• My code follows the project's style guidelines
• I have run pre-commit run --all-files and all checks pass
• I have added tests that prove my fix/feature works
• I have updated documentation if needed
• My changes don't introduce new warnings

---

Summary by cubic

Moves notification metrics from NotificationMetrics instruments to spans, with no change to the ten series emitted (part of #389).

Changes

• Each delivery is a notification.deliver span; the throttle, each retry, and each subscription update emit notification.throttled, notification.retry, and notification.subscription_change spans.
• The collector's signaltometrics connector derives the ten series from those spans, and label names become the span attribute names; the notifications dashboard moves with them.
• NotificationService now takes a trace.Tracer instead of a meter, and NotificationMetrics is removed.

^{Written for commit cec550a. Summary will update on new commits.}

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[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/app/services/notification_service.py	95.45%	1 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Flag	Coverage Δ
backend-e2e	82.81% <59.09%> (-0.03%)	⬇️
backend-unit	73.22% <95.45%> (+0.28%)	⬆️

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Files with missing lines	Coverage Δ
backend/app/core/metrics/__init__.py	100.00% <ø> (ø)
backend/app/core/providers.py	95.48% <ø> (-0.06%)	⬇️
backend/app/services/notification_service.py	67.20% <95.45%> (+1.87%)	⬆️

Components	Coverage Δ
Backend	92.02% <95.45%> (+0.06%)	⬆️
Frontend	92.60% <ø> (ø)

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