feat(dex): in-cluster Password CR client for local-password accounts

Writes passwords.dex.coreos.com CRs against the in-cluster Kubernetes
API using the pod's service-account token + cluster CA (no kubectl /
client-go dependency). NewPasswordClient returns ErrNotInCluster off
cluster so the web layer degrades gracefully in dev.

Load-bearing: Dex's kubernetes storage types Password.Hash as []byte,
which k8s JSON-marshals as base64 — so the `hash` field carries the
base64 of the bcrypt string, not the raw string. Storing the raw string
makes Dex's base64-decode-on-login produce garbage and every login fail.

409 -> ErrPasswordExists, 401/403 -> ErrForbidden (RBAC missing) so the
handler can give precise messages. Tested against an httptest TLS server.

bcrypt cost-12 hashing lives in the web handler; golang.org/x/crypto was
already a transitive dep (now promoted in go.sum).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.sum

@@ -91,6 +91,8 @@ go.opentelemetry.io/otel/trace v1.37.0 h1:HLdcFNbRQBE2imdSEgm/kwqmQj1Or1l/7bW6mx
 go.opentelemetry.io/otel/trace v1.37.0/go.mod h1:TlgrlQ+PtQO5XFerSPUYG0JSgGyryXewPGyayAWSBS0=
 go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
 go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
+golang.org/x/crypto v0.45.0 h1:jMBrvKuj23MTlT0bQEOBcAE0mjg8mK9RXFhRH6nyF3Q=
+golang.org/x/crypto v0.45.0/go.mod h1:XTGrrkGJve7CYK7J8PEww4aY7gM3qMCElcJQ8n8JdX4=
 golang.org/x/oauth2 v0.36.0 h1:peZ/1z27fi9hUOFCAZaHyrpWG5lwe0RJEEEeH0ThlIs=
 golang.org/x/oauth2 v0.36.0/go.mod h1:YDBUJMTkDnJS+A4BP4eZBjCqtokkg1hODuPjwiGPO7Q=
 golang.org/x/sync v0.18.0 h1:kr88TuHDroi+UVf+0hZnirlk8o8T+4MrK6mr60WkH/I=