feat(alerting): server-side state+severity filters, ButtonGroup filter UI

Backend: `GET /environments/{envSlug}/alerts` now accepts optional multi-value
`state=…` and `severity=…` query params. Filters are pushed down to
PostgresAlertInstanceRepository, which appends `AND state::text = ANY(?)` /
`AND severity::text = ANY(?)` to the inbox query (null/empty = no filter).

`AlertInstanceRepository.listForInbox` gained a 7-arg overload; the old 5-arg
form is preserved as a default delegate so existing callers (evaluator,
AlertingFullLifecycleIT, PostgresAlertInstanceRepositoryIT) compile unchanged.
`InAppInboxQuery.listInbox` also has a new filtered overload.

UI: InboxPage severity filter migrated from `SegmentedTabs` (single-select,
no color cues) to `ButtonGroup` (multi-select with severity-coloured dots),
matching the topnavbar status-filter pattern. `useAlerts` forwards the
filters as query params and cache-keys on the filter tuple so each combo
is independently cached.

Unit + hook tests updated to the new contract (5 UI tests + 8 Java unit
tests passing). OpenAPI types regenerated from the fresh local backend.

ui/src/api/schema.d.ts

@@ -7213,6 +7213,8 @@ export interface operations {
             query: {
                 env: components["schemas"]["Environment"];
                 limit?: number;
+                state?: ("PENDING" | "FIRING" | "ACKNOWLEDGED" | "RESOLVED")[];
+                severity?: ("CRITICAL" | "WARNING" | "INFO")[];
             };
             header?: never;
             path?: never;