Add WebAuthn config; Docker single-mount

- Add [webauthn] section to all config examples
- Add active WebAuthn config to run/mcias.conf
- Update Dockerfile to use /srv/mcias single mount
- Add WebAuthn and TOTP sections to RUNBOOK.md
- Fix TOTP QR display (template.URL type)
- Add --force-rm to docker build in Makefile

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dist/mcias.conf.docker.example

@@ -48,3 +48,14 @@ threads = 4
 # Set it with: docker run -e MCIAS_MASTER_PASSPHRASE=your-passphrase ...
 # or with a Docker secret / Kubernetes secret.
 passphrase_env = "MCIAS_MASTER_PASSPHRASE"
+
+# ---------------------------------------------------------------------------
+# [webauthn] — FIDO2/WebAuthn passkey authentication (OPTIONAL)
+# ---------------------------------------------------------------------------
+# Uncomment to enable passwordless passkey login. Set rp_id to your domain
+# and rp_origin to the full HTTPS origin users access in their browser.
+#
+# [webauthn]
+# rp_id        = "auth.example.com"
+# rp_origin    = "https://auth.example.com"
+# display_name = "MCIAS"