Adds the service layer for activation metrics

What does this MR do and why?

Adds the service layer for activation metrics

Database Queries

Note: This is a new table (activation_metrics) with no production data yet. Query plans should be generated against Database Lab (postgres.ai) with seeded data. See preparation when adding or modifying queries.

1. .for_user(user) scope

SELECT "activation_metrics".*
FROM "activation_metrics"
WHERE "activation_metrics"."user_id" = ?

Query plan:

2. .for_namespace(namespace) scope

SELECT "activation_metrics".*
FROM "activation_metrics"
WHERE "activation_metrics"."namespace_id" = ?

Query plan:

3. .by_metric(:merged_mr) scope

SELECT "activation_metrics".*
FROM "activation_metrics"
WHERE "activation_metrics"."metric" = 0

Query plan:

4. .completed?(user_id:, metric:, namespace_id:)

SELECT 1 AS one
FROM "activation_metrics"
WHERE "activation_metrics"."user_id" = ?
  AND "activation_metrics"."metric" = 0
  AND "activation_metrics"."namespace_id" = ?
LIMIT 1

Query plan:

5. .record_for — find_by path

SELECT "activation_metrics".*
FROM "activation_metrics"
WHERE "activation_metrics"."user_id" = ?
  AND "activation_metrics"."namespace_id" = ?
  AND "activation_metrics"."metric" = 0
LIMIT 1

Query plan:

6. .record_for — insert path

INSERT INTO "activation_metrics" ("user_id", "namespace_id", "metric", "created_at", "updated_at")
VALUES (?, ?, 0, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
RETURNING "id"

Query plan:

7. Finder — default query

SELECT "activation_metrics".*
FROM "activation_metrics"
WHERE "activation_metrics"."user_id" = ?

Query plan:

Index usage summary

Query	Index Used
.for_user	unique_activation_metric_user_id_namespace_id_and_metric
.for_namespace	index_activation_metrics_on_namespace_id
.by_metric	unique_activation_metric_user_id_namespace_id_and_metric
.completed?	unique_activation_metric_user_id_namespace_id_and_metric (index-only scan)
.record_for (find)	unique_activation_metric_user_id_namespace_id_and_metric
.record_for (insert)	unique_activation_metric_user_id_namespace_id_and_metric (unique constraint, NULLS NOT DISTINCT)
Finder	unique_activation_metric_user_id_namespace_id_and_metric

Migration

Replaces the existing unique index with one that uses NULLS NOT DISTINCT (PostgreSQL 15+) to enforce uniqueness even when namespace_id is NULL:

• Updated: unique_activation_metric_user_id_namespace_id_and_metric on (user_id, namespace_id, metric) — added NULLS NOT DISTINCT to treat NULL namespace_id as equal for uniqueness. Same index name retained, also provides FK index coverage for user_id

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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Related to #584191