feat: add Django backend foundation and Docker runtime

Add a Django admin-backed management layer for pobsync configs, runs,
snapshots, and schedules. Keep the existing CLI engine as the execution
source of truth, add import/run management commands, and provide SQLite
default plus optional MariaDB Docker Compose support.

docker-compose.yml

@@ -0,0 +1,58 @@
+services:
+  web:
+    build: .
+    command: python manage.py runserver 0.0.0.0:8000
+    environment:
+      POBSYNC_DJANGO_DEBUG: "1"
+      POBSYNC_DJANGO_SECRET_KEY: "dev-only-change-me"
+      POBSYNC_DJANGO_ALLOWED_HOSTS: "localhost,127.0.0.1,0.0.0.0"
+      POBSYNC_HOME: "/opt/pobsync"
+      POBSYNC_SQLITE_PATH: "/var/lib/pobsync/pobsync.sqlite3"
+    ports:
+      - "8000:8000"
+    volumes:
+      - pobsync_state:/opt/pobsync
+      - pobsync_db:/var/lib/pobsync
+
+  web-mariadb:
+    profiles: ["mariadb"]
+    build: .
+    command: python manage.py runserver 0.0.0.0:8000
+    environment:
+      POBSYNC_DJANGO_DEBUG: "1"
+      POBSYNC_DJANGO_SECRET_KEY: "dev-only-change-me"
+      POBSYNC_DJANGO_ALLOWED_HOSTS: "localhost,127.0.0.1,0.0.0.0"
+      POBSYNC_HOME: "/opt/pobsync"
+      POBSYNC_DB_ENGINE: "mariadb"
+      POBSYNC_DB_HOST: "db"
+      POBSYNC_DB_NAME: "pobsync"
+      POBSYNC_DB_USER: "pobsync"
+      POBSYNC_DB_PASSWORD: "pobsync"
+    depends_on:
+      db:
+        condition: service_healthy
+    ports:
+      - "8000:8000"
+    volumes:
+      - pobsync_state:/opt/pobsync
+
+  db:
+    profiles: ["mariadb"]
+    image: mariadb:11
+    environment:
+      MARIADB_DATABASE: "pobsync"
+      MARIADB_USER: "pobsync"
+      MARIADB_PASSWORD: "pobsync"
+      MARIADB_ROOT_PASSWORD: "pobsync-root"
+    volumes:
+      - mariadb_data:/var/lib/mysql
+    healthcheck:
+      test: ["CMD", "healthcheck.sh", "--connect", "--innodb_initialized"]
+      interval: 5s
+      timeout: 5s
+      retries: 20
+
+volumes:
+  pobsync_state:
+  pobsync_db:
+  mariadb_data: