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/dbgen/dbgen.go

@@ -468,7 +468,7 @@ func BoundarySession(t testing.TB, db database.Store, seed database.BoundarySess
 	session, err := db.InsertBoundarySession(genCtx, database.InsertBoundarySessionParams{
 		ID:                  takeFirst(seed.ID, uuid.New()),
 		WorkspaceAgentID:    takeFirst(seed.WorkspaceAgentID, uuid.New()),
-		OwnerID:             takeFirst(seed.OwnerID, uuid.NullUUID{UUID: uuid.New(), Valid: true}),
+		OwnerID:             seed.OwnerID,
 		ConfinedProcessName: takeFirst(seed.ConfinedProcessName, "claude-code"),
 		StartedAt:           takeFirst(seed.StartedAt, dbtime.Now()),
 		UpdatedAt:           takeFirst(seed.UpdatedAt, dbtime.Now()),