perf: improve routing insights view (2)

[eunjae-lee]

What does this PR do?

This is a follow-up of #18557. It removes assignment_reasons_agg and moves it into a subquery.

• Fixes #XXXX (GitHub issue number)
• Fixes CAL-XXXX (Linear issue number - should be visible at the bottom of the GitHub issue description)

diff of the two migrations:

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• N/A - I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

[linear]

CAL-4995 Insights routing - performance issue

[vercel]

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2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
cal	⬜️ Ignored (Inspect)	Visit Preview		Jan 9, 2025 4:59pm
calcom-web-canary	⬜️ Ignored (Inspect)	Visit Preview		Jan 9, 2025 4:59pm

[graphite-app]

Graphite Automations

"Add consumer team as reviewer" took an action on this PR • (01/09/25)

1 reviewer was added to this PR based on Keith Williams's automation.

[keithwillcode]

Ran migration locally and opened report. Everything ran fine.

[keithwillcode]

Also tested underlying query against prod. Good perf.

[github-actions]

E2E results are ready!

• Workflow #44014.1 latest results