(feature) add queued backup worker foundation

Move backup execution out of the management command into a reusable
backup runner service that can execute an existing BackupRun record.

Add queue primitives and a run_pobsync_worker command so manual backup
requests can be recorded as queued SQL state and processed outside the
web request path.

Add a worker Docker service and pobsync worker CLI alias, with tests for
queued run creation, worker execution, manual run typing, and command
mapping.

docker-compose.yml

@@ -29,6 +29,20 @@ services:
       - pobsync_db:/var/lib/pobsync
       - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
 
+  worker:
+    build: .
+    command: python manage.py run_pobsync_worker --loop --interval 15
+    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"
+    volumes:
+      - pobsync_state:/opt/pobsync
+      - pobsync_db:/var/lib/pobsync
+      - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
+
   web-mariadb:
     profiles: ["mariadb"]
     build: .
@@ -73,6 +87,27 @@ services:
       - pobsync_state:/opt/pobsync
       - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
 
+  worker-mariadb:
+    profiles: ["mariadb"]
+    build: .
+    command: python manage.py run_pobsync_worker --loop --interval 15
+    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
+    volumes:
+      - pobsync_state:/opt/pobsync
+      - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
+
   db:
     profiles: ["mariadb"]
     image: mariadb:11