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>

element/config/config.json.template

@@ -0,0 +1,39 @@
+{
+    "default_server_config": {
+        "m.homeserver": {
+            "base_url": "https://${MATRIX_SUBDOMAIN}.${DOMAIN}",
+            "server_name": "${DOMAIN}"
+        }
+    },
+    "default_server_name": "${DOMAIN}",
+    "disable_custom_urls": false,
+    "disable_guests": true,
+    "disable_login_language_selector": false,
+    "disable_3pid_login": false,
+    "force_verification": false,
+    "brand": "Element",
+    "integrations_ui_url": "",
+    "integrations_rest_url": "",
+    "integrations_widgets_urls": [],
+    "default_widget_container_height": 280,
+    "default_country_code": "FR",
+    "show_labs_settings": false,
+    "features": {},
+    "default_federate": false,
+    "default_theme": "light",
+    "room_directory": {
+        "servers": []
+    },
+    "enable_presence_by_hs_url": {},
+    "setting_defaults": {
+        "breadcrumbs": true
+    },
+    "jitsi": {
+        "preferred_domain": "meet.element.io"
+    },
+    "element_call": {
+        "brand": "Element Call",
+        "disable": false,
+        "use_exclusively": false
+    }
+}