feat(backend): replace backend_repo_index_job_completed with backend_repo_first_indexed

[brendan-kellam]

Summary

• Removes the backend_repo_index_job_completed PostHog event, which was firing on every re-index and generating enormous event volume (2M+ events/week, dominated by a single looping install)
• Adds backend_repo_first_indexed which fires only the first time a repo is successfully indexed (indexedAt === null before the update), enabling accurate tracking of unique repositories ever indexed

Test plan

• Deploy and verify backend_repo_first_indexed fires when a new repo is indexed for the first time
• Verify the event does not fire on subsequent re-indexes of the same repo
• Confirm backend_repo_index_job_completed no longer appears in PostHog

🤖 Generated with Claude Code

Summary by CodeRabbit

Changed

• Repository indexing event tracking refined to record first-time index completion separately, providing more precise analytics insights.

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[coderabbitai]

Walkthrough

The pull request replaces the general backend_repo_index_job_completed telemetry event with backend_repo_first_indexed, which fires conditionally only on the first successful repository index. The event payload is simplified by removing the jobType field.

Changes

Cohort / File(s)	Summary
Documentation CHANGELOG.md	Added entry documenting the replacement of backend_repo_index_job_completed with backend_repo_first_indexed event that fires only on first successful repo index.
Telemetry Event Definition packages/backend/src/posthogEvents.ts	Renamed event map entry from backend_repo_index_job_completed to backend_repo_first_indexed and removed the jobType field from its schema, keeping only repoId and type.
Event Emission Logic packages/backend/src/repoIndexManager.ts	Modified onJobCompleted to conditionally emit backend_repo_first_indexed only for INDEX-type jobs without a prior indexedAt timestamp, replacing the previous unconditional event emission on all job completions.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• #878: Introduced the original backend_repo_index_job_completed telemetry event that this PR modifies and replaces.
• #860: Modified repoIndexManager.ts job-completion handling logic that this PR's conditional event emission depends on.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: replacing one PostHog event with another. It directly reflects the primary objective of the PR.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch brendan/posthog-repo-indexing-events

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[coderabbitai]

🧹 Nitpick comments (1)

packages/backend/src/repoIndexManager.ts (1)

578-583: Add an inline comment to document the intentional stale-value read.

jobData.repo.indexedAt is the value fetched at line 514 — before db.repo.update sets indexedAt: new Date() at line 547. This is the correct sentinel for "first ever successful index", but the implicit ordering dependency is easy to accidentally break during future refactoring (e.g., merging the two db.repo.update calls).

📝 Suggested comment

+            // jobData.repo.indexedAt holds the value *before* the db.repo.update
+            // above set it to `new Date()`. A null value here means this is the
+            // first time this repo has been successfully indexed.
             if (jobData.type === RepoIndexingJobType.INDEX && jobData.repo.indexedAt === null) {
                 captureEvent('backend_repo_first_indexed', {
                     repoId: job.data.repoId,
                     type: jobData.repo.external_codeHostType,
                 });
             }

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@packages/backend/src/repoIndexManager.ts` around lines 578 - 583, Add an
inline comment at the check that reads jobData.repo.indexedAt === null (used
with RepoIndexingJobType.INDEX/jobData) explaining this is an intentional
stale-value read: jobData.repo.indexedAt was fetched earlier (when jobData was
built) and is used as the sentinel for "first ever successful index" even though
db.repo.update later sets indexedAt; mention the ordering dependency and warn
that merging or reordering the db.repo.update calls (or changing where jobData
is sourced) would break this logic so future maintainers should preserve the
read-before-update pattern.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@packages/backend/src/repoIndexManager.ts`:
- Around line 578-583: Add an inline comment at the check that reads
jobData.repo.indexedAt === null (used with RepoIndexingJobType.INDEX/jobData)
explaining this is an intentional stale-value read: jobData.repo.indexedAt was
fetched earlier (when jobData was built) and is used as the sentinel for "first
ever successful index" even though db.repo.update later sets indexedAt; mention
the ordering dependency and warn that merging or reordering the db.repo.update
calls (or changing where jobData is sourced) would break this logic so future
maintainers should preserve the read-before-update pattern.