Add Alerts webapp UI (ticket 020)

New Alerts panel: lists rules with condition summaries and an enabled
toggle, and a condition-type-specific editor (Immediate/Threshold/NewValue/
Ratio) with client-side validation before save, reusing FilterBar's query
input styling for query-based conditions. Adds a test-notification endpoint
(POST /api/settings/alerts/test) so an ntfy topic/server can be verified
without saving a rule first, plus a docs section and help icons matching
ticket 021's pattern.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

tickets/020-alerts-webapp-ui.md

@@ -1,41 +0,0 @@
-# 020 — Alerts webapp UI
-
-## Summary
-
-A webapp page for managing `AlertRule`s ([017](017-ntfy-alerts.md)) end to
-end: create/edit/enable/delete rules and configure their ntfy sink, without
-touching `settings.json` by hand.
-
-## Scope
-
-- A new route/panel (matching how `Dashboards`/`Metrics` are already surfaced
-  in `App.tsx`) listing existing alert rules: name, condition summary,
-  enabled state.
-- A rule editor:
-  - Condition type picker (Immediate / Threshold / New value / Ratio, see
-    [018](018-alert-trigger-evaluation.md)) with type-specific fields, reusing
-    `FilterBar`'s query input for the query-based conditions rather than a
-    separate text field.
-  - Ntfy sink fields: topic and optional server override.
-  - Enabled toggle.
-- "Send test notification" button wired to the sink delivery endpoint from
-  [019](019-ntfy-sink-delivery.md), so a topic/server can be verified without
-  waiting for a real trigger.
-- Contextual "?" help icons on the condition-type picker and sink fields,
-  matching [021](021-docs-contextual-help.md)'s pattern.
-
-## Acceptance criteria
-
-- Rules created/edited/deleted in the UI persist across a page reload (i.e.
-  round-trip through the CRUD endpoints from 017).
-- Each condition type's fields validate before save (e.g. Ratio requires two
-  queries and a threshold).
-- "Send test notification" delivers to the currently-entered topic/server
-  without requiring the rule to be saved first.
-
-## Depends on
-
-- [017 — Alert definitions & settings persistence](017-ntfy-alerts.md)
-- [018 — Alert trigger evaluation](018-alert-trigger-evaluation.md)
-- [019 — Ntfy sink delivery](019-ntfy-sink-delivery.md)
-- [021 — Docs contextual help](021-docs-contextual-help.md)