feat: make Django configs drive backups and scheduling

Treat SQL-backed Django models as the source of truth for pobsync
configuration, exporting runtime YAML only as a compatibility layer for
the existing engine. Add a database-driven scheduler command, Docker
scheduler services, schedule run-state fields, and tests for scheduler,
config export, and retention behavior.

docker-compose.yml

@@ -14,6 +14,19 @@ services:
       - pobsync_state:/opt/pobsync
       - pobsync_db:/var/lib/pobsync
 
+  scheduler:
+    build: .
+    command: python manage.py run_pobsync_scheduler --loop --interval 60
+    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
+
   web-mariadb:
     profiles: ["mariadb"]
     build: .
@@ -36,6 +49,26 @@ services:
     volumes:
       - pobsync_state:/opt/pobsync
 
+  scheduler-mariadb:
+    profiles: ["mariadb"]
+    build: .
+    command: python manage.py run_pobsync_scheduler --loop --interval 60
+    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
+
   db:
     profiles: ["mariadb"]
     image: mariadb:11