feat: purge boundary logs past retention (#24815)

Add a periodic purge job for `boundary_logs` rows past their retention
threshold, following the same pattern as the existing audit log and
connection log purge jobs in `dbpurge`.

Expose a `--boundary-log-retention` deployment flag (env
`CODER_BOUNDARY_LOG_RETENTION`, YAML `retention.boundary_logs`). Default
is `0` (keep indefinitely). When set to a positive duration, `purgeTick`
deletes rows where `captured_at` is older than the threshold in batches
of 10,000, matching other log purge operations. The `boundary_logs`
label is added to the `records_purged_total` Prometheus counter.

Also removes the random-UUID fallback for `OwnerID` in
`dbgen.BoundarySession`. The previous fallback generated a UUID that
could never satisfy the `boundary_sessions_owner_id_fkey` FK constraint,
masking test setup bugs. Callers must now provide a valid user ID or
accept NULL (the legitimate "user deleted" state).

coderd/database/queries/boundarylogs.sql

@@ -77,3 +77,20 @@ WITH old_logs AS (
 DELETE FROM boundary_logs
 USING old_logs
 WHERE boundary_logs.id = old_logs.id;
+
+-- name: DeleteOldBoundarySessions :execrows
+-- Deletes boundary sessions that have aged past retention and no longer
+-- have any associated logs.
+WITH old_sessions AS (
+    SELECT bs.id
+    FROM boundary_sessions bs
+    WHERE bs.updated_at < @before_time::timestamptz
+      AND NOT EXISTS (
+          SELECT 1 FROM boundary_logs bl WHERE bl.session_id = bs.id
+      )
+    ORDER BY bs.updated_at ASC
+    LIMIT @limit_count
+)
+DELETE FROM boundary_sessions
+USING old_sessions
+WHERE boundary_sessions.id = old_sessions.id;