feat(inbox): Allow filtering reports by product (#1421)

## Problem

No way to filter inbox reports by signal source - you see all reports regardless of whether they came from session replay, error tracking, GitHub issues, etc.

## Changes

Adding "Source" multi-select in the toolbar filter popover (session replay, error tracking, LLM analytics, GitHub, Linear, Zendesk).

Backend support (in posthog/ `sig/add-signal-report-task` branch) queries ClickHouse `document_embeddings` for report IDs that have signals from the requested source products, then filters the PostgreSQL queryset by those IDs.

Author: Twixes

apps/code/src/renderer/api/posthogClient.ts

@@ -953,6 +953,9 @@ export class PostHogAPIClient {
     if (params?.ordering) {
       url.searchParams.set("ordering", params.ordering);
     }
+    if (params?.source_product) {
+      url.searchParams.set("source_product", params.source_product);
+    }
 
     const response = await this.api.fetcher.fetch({
       method: "get",