Initial commit: Matrix stack (Synapse + Synapse-Admin + Element Web) behind Traefik

Fully dockerized deployment with automatic Let's Encrypt certificates via
Traefik's static file provider (no docker.sock needed), bind-mounted data
folders instead of named volumes, and domain/secrets kept out of git via
.env + rendered templates.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

wellknown/well-known-matrix-client.json.template

@@ -0,0 +1,5 @@
+{
+    "m.homeserver": {
+        "base_url": "https://${MATRIX_SUBDOMAIN}.${DOMAIN}"
+    }
+}