feat(#384): Add Synapse + Element Web to docker-compose for dev

- Create docker-compose.matrix.yml as optional dev overlay
- Add Synapse homeserver config with shared PostgreSQL
- Add Element Web client config (port 8501)
- Add bot account setup script (docker/matrix/scripts/setup-bot.sh)
- Add Makefile targets: matrix-up, matrix-down, matrix-logs, matrix-setup-bot
- Document Matrix env vars in .env.example
- Synapse accessible at localhost:8008, Element at localhost:8501
- Usage: docker compose -f docker/docker-compose.yml -f docker/docker-compose.matrix.yml up

Refs #384

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

docker/matrix/element/config.json

@@ -0,0 +1,30 @@
+{
+  "default_server_config": {
+    "m.homeserver": {
+      "base_url": "http://localhost:8008",
+      "server_name": "localhost"
+    }
+  },
+  "brand": "Mosaic Stack Dev",
+  "default_theme": "dark",
+  "room_directory": {
+    "servers": ["localhost"]
+  },
+  "features": {
+    "feature_video_rooms": false,
+    "feature_group_calls": false
+  },
+  "show_labs_settings": true,
+  "piwik": false,
+  "posthog": {
+    "enabled": false
+  },
+  "privacy_policy_url": null,
+  "terms_and_conditions_links": [],
+  "setting_defaults": {
+    "breadcrumbs": true,
+    "custom_themes": []
+  },
+  "disable_guests": true,
+  "disable_3pid_login": true
+}