feat(backend): replace backend_repo_index_job_completed with backend_repo_first_indexed

Replaces the high-volume per-index PostHog event with one that fires only
on the first successful index of a repo, enabling accurate tracking of
unique repositories ever indexed without flooding the event stream.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: brendan-kellam

packages/backend/src/posthogEvents.ts

@@ -19,9 +19,8 @@ export type PosthogEventMap = {
         jobType: 'INDEX' | 'CLEANUP',
         type: string,
     },
-    backend_repo_index_job_completed: {
+    backend_repo_first_indexed: {
         repoId: number,
-        jobType: 'INDEX' | 'CLEANUP',
         type: string,
     },
 }

packages/backend/src/repoIndexManager.ts

@@ -575,11 +575,12 @@ export class RepoIndexManager {
             this.promClient.activeRepoIndexJobs.dec({ repo: job.data.repoName, type: jobTypeLabel });
             this.promClient.repoIndexJobSuccessTotal.inc({ repo: job.data.repoName, type: jobTypeLabel });
 
-            captureEvent('backend_repo_index_job_completed', {
-                repoId: job.data.repoId,
-                jobType: job.data.type,
-                type: jobData.repo.external_codeHostType,
-            });
+            if (jobData.type === RepoIndexingJobType.INDEX && jobData.repo.indexedAt === null) {
+                captureEvent('backend_repo_first_indexed', {
+                    repoId: job.data.repoId,
+                    type: jobData.repo.external_codeHostType,
+                });
+            }
         } catch (error) {
             Sentry.captureException(error);
             logger.error(`Exception thrown while executing lifecycle function \`onJobCompleted\`.`, error);