perf: optimize prebuilds membership reconciliation to check orgs not presets (#20493)

## Description

The membership reconciliation ensures the prebuilds system user is a
member of all organizations with prebuilds configured. To support
prebuilds quota management, each organization must have a prebuilds
group that the system user belongs to.

## Problem

Previously, membership reconciliation iterated over all presets to check
and update membership status. This meant database queries
`GetGroupByOrgAndName` and `InsertGroupMember` were executed for each
preset. Since presets are unique combinations of `(organization,
template, template version, preset)`, this resulted in several redundant
checks for the same organization.

In dogfood, `InsertGroupMember` was called thousands of times per day,
even though memberships were already configured ([internal Grafana
dashboard link](https://grafana.dev.coder.com/goto/46MZ1UgDg?orgId=1))

<img width="5382" height="1788" alt="Screenshot 2025-10-28 at 16 01 36"
src="https://github.com/user-attachments/assets/757b7253-106f-4f72-8586-8e2ede9f18db"
/>

## Solution

This PR introduces `GetOrganizationsWithPrebuildStatus`, a single query
that returns:
* All unique organizations with prebuilds configured
* Whether the prebuilds user is a member of each organization
* Whether the prebuilds group exists in each organization
* Whether the prebuilds user is in the prebuilds group

The membership reconciliation logic now:
* Fetches status for all organizations in one query
* Only performs inserts for organizations missing required memberships
or groups
* Safely handles concurrent operations via unique constraint violations
* This reduces database load from `O(presets)` to `O(organizations)` per
reconciliation loop, with a single read query when everything is
configured.

## Changes

* Add `GetOrganizationsWithPrebuildStatus` SQL query
* Update `membership.ReconcileAll` to use organization-based
reconciliation instead of preset-based
* Update tests to reflect new behavior

Related to internal thread:
https://codercom.slack.com/archives/C07GRNNRW03/p1760535570381369

coderd/database/queries/prebuilds.sql

@@ -327,3 +327,47 @@ SET
 FROM jobs_to_cancel
 WHERE provisioner_jobs.id = jobs_to_cancel.id
 RETURNING jobs_to_cancel.id, jobs_to_cancel.workspace_id, jobs_to_cancel.template_id, jobs_to_cancel.template_version_preset_id;
+
+-- name: GetOrganizationsWithPrebuildStatus :many
+-- GetOrganizationsWithPrebuildStatus returns organizations with prebuilds configured and their
+-- membership status for the prebuilds system user (org membership, group existence, group membership).
+WITH orgs_with_prebuilds AS (
+	-- Get unique organizations that have presets with prebuilds configured
+	SELECT DISTINCT o.id, o.name
+	FROM organizations o
+	INNER JOIN templates t ON t.organization_id = o.id
+	INNER JOIN template_versions tv ON tv.template_id = t.id
+	INNER JOIN template_version_presets tvp ON tvp.template_version_id = tv.id
+	WHERE tvp.desired_instances IS NOT NULL
+),
+prebuild_user_membership AS (
+	-- Check if the user is a member of the organizations
+	SELECT om.organization_id
+	FROM organization_members om
+	INNER JOIN orgs_with_prebuilds owp ON owp.id = om.organization_id
+	WHERE om.user_id = @user_id::uuid
+),
+prebuild_groups AS (
+	-- Check if the organizations have the prebuilds group
+	SELECT g.organization_id, g.id as group_id
+	FROM groups g
+	INNER JOIN orgs_with_prebuilds owp ON owp.id = g.organization_id
+	WHERE g.name = @group_name::text
+),
+prebuild_group_membership AS (
+	-- Check if the user is in the prebuilds group
+	SELECT pg.organization_id
+	FROM prebuild_groups pg
+	INNER JOIN group_members gm ON gm.group_id = pg.group_id
+	WHERE gm.user_id = @user_id::uuid
+)
+SELECT
+	owp.id AS organization_id,
+	owp.name AS organization_name,
+	(pum.organization_id IS NOT NULL)::boolean AS has_prebuild_user,
+	pg.group_id AS prebuilds_group_id,
+	(pgm.organization_id IS NOT NULL)::boolean AS has_prebuild_user_in_group
+FROM orgs_with_prebuilds owp
+LEFT JOIN prebuild_groups pg ON pg.organization_id = owp.id
+LEFT JOIN prebuild_user_membership pum ON pum.organization_id = owp.id
+LEFT JOIN prebuild_group_membership pgm ON pgm.organization_id = owp.id;