Chore: refactor PR histogram bucket generation query

[jpmcb]

The PR time_bucket query to timescale is not performant:

this is a screenshot from some insights on timescale we can see and the medium time is way too slow. Which is surprising since it's using a hypertable and should be reporting faster results. There may be outliers but if I had to guess, it's because we build a partition table to then build the time series from in the genPrHistogram function:

api/src/timescale/pull_request_github_events.service.ts

Lines 498 to 504 in 5f9897c

	const cteBuilder = this.pullRequestGithubEventsRepository
	.createQueryBuilder("pull_request_github_events")
	.select("*")
	.addSelect(`ROW_NUMBER() OVER (PARTITION BY pr_number, repo_name ORDER BY event_time ${order}) AS row_num`)
	.orderBy("event_time", order)
	.where(`'${startDate}'::TIMESTAMP >= "pull_request_github_events"."event_time"`)
	.andWhere(`'${startDate}'::TIMESTAMP - INTERVAL '${range} days' <= "pull_request_github_events"."event_time"`);

Regardless, this needs a second look.