Add filters, server-side sort, and test incident hiding to incidents tables

[kwent]

Summary

Adds filters, server-side sorting, and test incident hiding to the incidents tables (RootlyPage Incidents tab and RootlyIncidentsPage entity tabs).

Closes #47
Closes #48
Closes #49

Filters (#47)

• New filter bar above the table: Status, Severity, Teams, Services, Environments (multi-select autocompletes, options searched through the Rootly API), plus Clear filters.
• Filters are sent to GET /v1/incidents as filter[status], filter[severity_id], filter[team_ids], filter[service_ids], filter[environment_ids], so they apply across every page.
• Filters a page already fixes through params are hidden. The Rootly API ORs all relationship filters (environments, services, functionalities, teams, ...) together, so on entity pages scoped to a service, functionality, team or system the team/service/environment filters are hidden: adding one would widen the results instead of narrowing them. On the global page, a caption explains that several relationship filters match any selected item.
• Ongoing-incident tables on entity pages pass showFilters={false} to stay compact.

Server-side sort (#48)

• The table now uses Material Table remote data: page, page size and sort are sent to the Rootly API (sort=-started_at by default).
• The Started At, Mitigated At (new) and Resolved At (new) columns are sortable. The API only sorts on created_at, updated_at, started_at, in_triage_at, mitigated_at and resolved_at, so the other columns are no longer clickable. Before, clicking them only re-sorted the current page.
• Changing search, filters or the test toggle refetches from page 1 and keeps the current sort.

Test incidents (#49)

• Test incidents are hidden by default with filter[kind][not_in]=test,test_sub.
• New per-organization config rootly.<org>.showTestIncidents (@visibility frontend) lists them by default, for example in a sandbox account.
• A Show test incidents toggle in the filter bar overrides the default for the session.

Also

• Fixes the functionality incidents tables sending filter[functionalitys] and the system tables sending filter[groups]. Neither is a supported filter, so those tables were not scoped. They now send functionalities and teams.
• Search input is debounced (300ms) before it reaches the API.
• resolveRootlyOrganizationKey / useRootlyInstanceSettings helpers in src/api.ts resolve the active rootly.<key> block (explicit, then isDefault, then the first entry).
• Fixes a race in the overview card test: the card briefly renders "No ongoing incidents" before the resource loads, which made the test flaky.
• README documents the configuration options, test incidents and the filter/sort behavior. CHANGELOG has an Unreleased entry.

Notes

• @rootly/backstage-plugin-common has no public severities or environments methods yet. Those two option lists go through the client's authenticated fetch (/v1/severities, /v1/environments), with a guard that returns no options if it is unavailable. Follow-up: add getSeverities / getEnvironments to the common package.
• RootlyIncidentsFetchOpts has no sort field. The extra sort key goes through the common client's qs.stringify unchanged.

Test plan

• yarn tsc
• yarn lint (no new errors; 2 new warnings for import React, same pattern as the rest of the codebase)
• yarn test: 134 tests pass, including 10 consecutive full runs
• New unit tests for query building, filter options, the filter bar, instance settings and the config schema
• Integration test against the real Backstage/Material Table remote data (sort header clicks, sort kept across pages)