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-dev.conf.example

@@ -41,3 +41,10 @@ threads = 4
 
 [master_key]
 passphrase_env = "MCIAS_MASTER_PASSPHRASE"
+
+# WebAuthn — passkey authentication for local development.
+# rp_origin includes the non-standard port since we're not behind a proxy.
+[webauthn]
+rp_id        = "localhost"
+rp_origin    = "https://localhost:8443"
+display_name = "MCIAS (dev)"