feat: report user secrets adoption summary in telemetry (#24854)

Add a deployment-wide user secrets summary to the telemetry snapshot so
we can track adoption of user secrets
The summary reports:

- A breakdown of secrets by which injection fields are populated:
EnvNameOnly, FilePathOnly, Both, Neither
- The distribution of secrets per user (max, p25, p50, p75, p90)

All metrics are scoped to active non-system users. Soft-deleted users
are excluded. The percentile distribution is computed across the entire
active non-system user base, including users with zero secrets, so the
percentiles reflect deployment-wide adoption.

Assisted by Coder Agents.

coderd/database/migrations/testdata/fixtures/000486_user_secrets_telemetry_lock.up.sql

@@ -0,0 +1,3 @@
+-- Smoke fixture: a single user_secrets_summary lock for a fixed period.
+INSERT INTO telemetry_locks (event_type, period_ending_at)
+VALUES ('user_secrets_summary', '2026-01-01 00:00:00+00');