feat: Business Central backup Docker container with web management panel

Python backup module (Azure AD OAuth2 → BC API v2.0 → GPG AES256 → S3),
Flask web UI on port 1890 with SSE live log streaming, dual incremental +
full backup schedules, chain restore merging full + incrementals, and
per-entity file browser with ZIP download.

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

docker-compose.yml

@@ -0,0 +1,16 @@
+services:
+  bcbak:
+    build: .
+    container_name: bcbak
+    restart: unless-stopped
+    ports:
+      - "1890:1890"
+    volumes:
+      - bcbak_data:/data
+    environment:
+      - BCBAK_DATA=/data
+      - PORT=1890
+      - SECRET_KEY=${SECRET_KEY:-change-me-in-production}
+
+volumes:
+  bcbak_data: